U.S. patent number 8,828,574 [Application Number 13/475,324] was granted by the patent office on 2014-09-09 for electrolyte compositions for aqueous electrolyte lithium sulfur batteries.
This patent grant is currently assigned to PolyPlus Battery Company. The grantee listed for this patent is Lutgard C. De Jonghe, Nikolay Goncharenko, Bruce D. Katz, Valentina Loginova, Yevgeniy S. Nimon, Steven J. Visco. Invention is credited to Lutgard C. De Jonghe, Nikolay Goncharenko, Bruce D. Katz, Valentina Loginova, Yevgeniy S. Nimon, Steven J. Visco.
United States Patent |
8,828,574 |
Visco , et al. |
September 9, 2014 |
**Please see images for:
( Certificate of Correction ) ** |
Electrolyte compositions for aqueous electrolyte lithium sulfur
batteries
Abstract
Provided are lithium sulfur battery cells that use water as an
electrolyte solvent. In various embodiments the water solvent
enhances one or more of the following cell attributes: energy
density, power density and cycle life. Significant cost reduction
can also be realized by using an aqueous electrolyte in combination
with a sulfur cathode. For instance, in applications where cost per
Watt-Hour (Wh) is paramount, such as grid storage and traction
applications, the use of an aqueous electrolyte in combination with
inexpensive sulfur as the cathode active material can be a key
enabler for the utility and automotive industries, providing a cost
effective and compact solution for load leveling, electric vehicles
and renewable energy storage.
Inventors: |
Visco; Steven J. (Berkeley,
CA), Nimon; Yevgeniy S. (Danville, CA), Katz; Bruce
D. (Moraga, CA), De Jonghe; Lutgard C. (Lafayette,
CA), Goncharenko; Nikolay (Walnut Creek, CA), Loginova;
Valentina (Walnut Creek, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Visco; Steven J.
Nimon; Yevgeniy S.
Katz; Bruce D.
De Jonghe; Lutgard C.
Goncharenko; Nikolay
Loginova; Valentina |
Berkeley
Danville
Moraga
Lafayette
Walnut Creek
Walnut Creek |
CA
CA
CA
CA
CA
CA |
US
US
US
US
US
US |
|
|
Assignee: |
PolyPlus Battery Company
(Berkeley, CA)
|
Family
ID: |
48280944 |
Appl.
No.: |
13/475,324 |
Filed: |
May 18, 2012 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20130122334 A1 |
May 16, 2013 |
|
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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13440847 |
Apr 5, 2012 |
|
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61585589 |
Jan 11, 2012 |
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61560134 |
Nov 15, 2011 |
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61623031 |
Apr 11, 2012 |
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Current U.S.
Class: |
429/105 |
Current CPC
Class: |
H01M
4/405 (20130101); H01M 4/5815 (20130101); H01M
4/382 (20130101); H01M 4/38 (20130101); Y10T
29/49108 (20150115); H01M 6/045 (20130101); Y02E
60/10 (20130101) |
Current International
Class: |
H01M
4/38 (20060101) |
Field of
Search: |
;429/105 |
References Cited
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Apr 2005 |
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WO |
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|
Primary Examiner: Ruddock; Ula C
Assistant Examiner: Chmielecki; Scott J
Attorney, Agent or Firm: Weaver Austin Villeneuve &
Sampson LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. patent
application Ser. Nos. 13/440,847, filed Apr. 5, 2012, titled
AQUEOUS ELECTROLYTE LITHIUM SULFUR BATTERIES; which claims priority
to U.S. Provisional Patent Application Nos. 61/585,589, filed Jan.
11, 2012, titled AQUEOUS LITHIUM-SULFUR BATTERY CELL, and
61/560,134, filed Nov. 15, 2011, titled AQUEOUS LITHIUM-SULFUR
BATTERY. This application also claims priority from U.S.
Provisional Patent Application Nos. 61/623,031, filed Apr. 11,
2012, titled AQUEOUS ELECTROLYTE LITHIUM SULFUR BATTERIES. Each of
these applications is incorporated herein by reference in its
entirety and for all purposes.
Claims
What is claimed is:
1. An aqueous lithium sulfur electrochemical cell comprising: an
anode structure comprising an electroactive material; a cathode
comprising a solid electron transfer medium; an aqueous electrolyte
comprising at least 10 vol % of a non-aqueous solvent and greater
than 10 vol % of water in direct contact with the electron transfer
medium; and active sulfur species in direct contact with the
aqueous electrolyte; wherein the anode electroactive material is
isolated from direct contact with the aqueous electrolyte.
2. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the amount of water in the aqueous electrolyte is greater
than 20 vol %.
3. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the amount of water in the aqueous electrolyte is greater
than 30%.
4. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the amount of water in the aqueous electrolyte is greater
than 40%.
5. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein water is the main solvent.
6. The aqueous lithium sulfur electrochemical cell of claim 5,
wherein the amount of non-aqueous solvent in the aqueous
electrolyte is between 10 and 20 vol %.
7. The aqueous lithium sulfur electrochemical cell of claim 5
wherein the amount of non-aqueous solvent in the aqueous
electrolyte is between 20 and 30 vol %.
8. The aqueous lithium sulfur electrochemical cell of claim 5,
wherein the amount on non-aqueous solvent in the aqueous
electrolyte is between 30 and 40 vol %.
9. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the amount of non-aqueous solvent in the aqueous
electrolyte is up to 80 vol %.
10. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the active sulfur concentration in the aqueous electrolyte
is greater than 8 molar sulfur.
11. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the active sulfur concentration in the aqueous electrolyte
is greater than 10 molar sulfur.
12. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the active sulfur concentration in the aqueous electrolyte
is greater than 11 molar sulfur.
13. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the active sulfur concentration in the aqueous electrolyte
is greater than 12 molar sulfur.
14. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the active lithium sulfur stoichiometric ratio in the
electrolyte is Li.sub.2S.sub.x(x>5).
15. The aqueous lithium sulfur electrochemical cell of claim 14,
wherein active sulfur concentration in the aqueous electrolyte is
between 10 and 17 molar.
16. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the non-aqueous solvent is a protic organic liquid
solvent.
17. The aqueous lithium sulfur electrochemical cell of claim 16,
wherein the solvent is an amine.
18. The aqueous lithium sulfur electrochemical cell of claim 17,
wherein the solvent is a diamine.
19. The aqueous lithium sulfur electrochemical cell of claim 16,
wherein the solvent is an alcohol.
20. The aqueous lithium sulfur electrochemical cell of claim 19,
wherein the alcohol is selected from the group consisting of
methanol, glycols and polyglycols.
21. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the non-aqueous solvent is aprotic.
22. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the non-aqueous solvent has a donor number between 15 to
40.
23. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the active sulfur concentration of the aqueous electrolyte
prior to initially operating the cell is selected from the group
consisting of a value that is greater than 5 molar sulfur, greater
than 6 molar sulfur, greater than 7 molar sulfur, greater than 8
molar sulfur, greater than 9 molar sulfur, greater than 10 molar
sulfur, greater than 11 molar sulfur, and greater than 12 molar
sulfur.
24. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the lithium electroactive material is lithium metal.
25. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the lithium electroactive material comprises a lithium
intercalation material.
26. The aqueous lithium sulfur electrochemical cell of claim 25,
wherein the lithium intercalation material is carbon.
27. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the lithium electroactive material comprises lithium alloy
material.
28. The aqueous lithium sulfur electrochemical cell of claim 27,
wherein the lithium alloy material comprises silicon.
29. The aqueous lithium sulfur electrochemical cell of claim 1,
wherein the cell is a secondary lithium battery cell.
Description
FIELD OF THE INVENTION
The present invention relates generally to the field of
electrochemical energy storage and power delivery. In particular,
the present invention is directed to aqueous lithium-sulfur battery
cells, including flow cells, and methods of making such cells.
BACKGROUND OF THE INVENTION
The lithium sulfur battery has a theoretical capacity of 1675
mAhg.sup.-1 and approximately 2300 Wh/kg. The low cost and
exceptionally high specific capacity of sulfur renders it an
especially attractive battery cathode material for large-scale
energy storage, including electric vehicle and grid storage
applications. Yet after more than twenty years of research and
development at various battery companies and scientific
institutions worldwide, key technical problems with the sulfur
electrode have precluded meaningful commercialization of the Li-S
battery.
SUMMARY OF THE INVENTION
In one aspect the invention provides an aqueous lithium sulfur
battery cell having an anode structure comprising an electroactive
material, a cathode comprising a solid electron transfer medium, an
aqueous electrolyte in contact with the electron transfer medium,
and active sulfur species in contact with the aqueous electrolyte,
and wherein the anode electroactive material is isolated from
direct contact with the aqueous electrolyte. Notably, while the
anode electroactive material is isolated from touching (i.e.,
directly contacting) the aqueous electrolyte, it is nonetheless
configured in the anode structure to be in lithium ion
communication with the aqueous electrolyte. Moreover, because the
aqueous electrolyte does not touch the anode electroactive material
but does directly contact the cathode the term "aqueous catholyte"
(or more simply "catholyte") is used interchangeably with the term
"aqueous electrolyte".
In various embodiments the aqueous electrolyte is electroactive in
that it contains dissolved active sulfur species that undergo
electrochemical redox at the cathode during discharge and charge.
Without limitation, the dissolved redox active sulfur species may
include sulfide anions (S.sup.2-), hydrosulfide anions (HS.sup.-),
and polysulfide anions including S.sub.x.sup.2- with x>1 and
hydropolysulfide anions (HS.sub.x.sup.- with x>1), and
combinations thereof.
In accordance with the present invention, the amount of water in
the catholyte is significant (i.e., not merely a trace amount). In
various embodiments the volume percent of water relative to the
total liquid solvent volume in the catholyte is greater than 5%,
10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and greater than 90%. In
certain embodiments water is the only liquid solvent in the
catholyte (i.e., water constitutes 100% of the solvent volume of
the catholyte). In various embodiments water is the main liquid
solvent in the catholyte. By use of the term main liquid solvent,
it is meant that the volume percent of water in the catholyte is
greater than the volume percent of any other liquid solvent.
Water has unique properties. In the aqueous sulfur catholyte
solutions described herein, the presence of water provides a number
of benefits, including high solubility for active sulfur species,
including lithium sulfide (Li.sub.2S), very high ionic conductivity
even at high sulfur concentrations, and fast dissolution kinetics.
The combination of high solubility, high conductivity, and fast
dissolution kinetics provides compelling lithium sulfur battery
performance.
Accordingly, in various embodiments the cell is fabricated with an
aqueous catholyte having a high concentration of active sulfur
species already dissolved therein. In other words, the cell has a
significant amount of dissolved active sulfur species adjacent the
electron transfer medium even before the cell has been initially
operated (e.g., initially discharged) and by this expedient the
fast electro-kinetics of solution phase redox can be used to
advantage, especially, but not exclusively, for applications that
require high current drain immediately upon start up. For instance,
in various embodiments, prior to initially operating the cell, the
active sulfur concentration in the aqueous electrolyte is greater
than 0.5 molar sulfur, 1 molar sulfur, 2 molar sulfur, 3 molar
sulfur, 4 molar sulfur, 5 molar sulfur, 6 molar sulfur, 7 molar
sulfur, 8 molar sulfur, 9 molar sulfur, 10 molar sulfur, 11 molar
sulfur, or greater than 12 molar sulfur. Herein and in the claims,
by the use of the term "molar sulfur" it is meant the number of
moles of sulfur per liter of electrolyte. Moreover, by use of the
phrase "just prior to initially operating the cell" or "prior to
initial cell operation" it is meant, herein and in the claims, to
mean the first (i.e., initial) electrochemical operation activated
by the user and specifically it refers to one or the other of cell
discharge or cell charge, whichever is caused to occur, by the
user, first. In other words, incidental self-discharge (e.g., on
storage) does not qualify herein or in the claims as an initial
cell operation.
Moreover, because it can be difficult to identify the precise
chemical nature of the various active sulfur species existing in
the catholyte solution, the composition of the active species in
the catholyte (i.e., active catholyte composition) is sometimes
expressed herein, and in the claims, in terms of an "active lithium
sulfur stoichiometric ratio" or more simply an "active
stoichiometric ratio" which is the ratio of active sulfur to active
lithium dissolved in the electrolyte, and represented by the
general formula Li.sub.2S.sub.x. Furthermore, it should be
understood that the "active stoichiometric ratio" as used herein is
exclusive of any non-active lithium salts and/or non-active sulfur
salts that may be added to the electrolyte for any purpose,
including, e.g., to enhance lithium ion conductivity in the case
of, e.g., a non-active LiCl salt, or a non-active sulfur containing
salt such as, e.g., LiSO.sub.3CF.sub.3.
Accordingly, in various embodiments, the active lithium sulfur
stoichiometric ratio in the catholyte prior to, in particular just
prior to, initial cell operation is Li.sub.2S; Li.sub.2S.sub.x
(x>1); Li.sub.2S.sub.x (1<x<5); Li.sub.2S.sub.5; or
Li.sub.2S.sub.x (x>5), and the concentration of the dissolved
active sulfur species is typically significant, e.g., greater than
1 molar sulfur. For instance, in particular embodiments, especially
for cells using a lithium metal or lithium alloy as the
electroactive anode material, the active stoichiometric ratio just
prior to initial cell operation is Li.sub.2S.sub.x with the
following range for x: 2.ltoreq.x.ltoreq.5, and the active sulfur
concentration is between 10 to 17 molar sulfur. For example, a
catholyte composition having an active stoichiometric ratio of
about Li.sub.2S.sub.4, and at concentrations greater than 10 molar
sulfur (e.g., 11, 12, 13, 14, 15, 16 or 17 molar sulfur). In
another particular embodiment, especially useful for cells which
are fabricated in the fully or mostly discharged state (e.g.,
having an anode electroactive material that is devoid of active
lithium), the active stoichiometric ratio of the catholyte just
prior to initial cell operation is Li.sub.2S, and the active sulfur
concentration is typically greater than 1 molar sulfur, and
preferably greater than 2 molar sulfur, and more preferably greater
than 3 molar sulfur (e.g., 3 molar, 4 molar, or 5 molar
sulfur).
Another advantage of the aqueous catholyte is that it may serve as
a medium into which high concentrations of fully or partially
reduced active sulfur species (e.g., Li.sub.2S) may be quickly
dissolved during charge. By this expedient high capacity cells in
accordance with embodiments of the instant invention may be deeply
discharged repeatedly since the cell reaction product on discharge
(e.g., Li.sub.2S) is readily dissolved and therefore more readily
oxidized on charge. Thus, in various embodiments, the cell is
formulated and operated such that a significant portion of the
sulfur ampere-hour capacity, at the end of discharge, is present in
the form of solid phase lithium sulfide.
Furthermore, the combination of high solubility and fast
dissolution kinetics of Li.sub.2S in water also enables a practical
method of making an aqueous lithium sulfur cell that is assembled
in the fully discharged state, and which makes use of alternative
anode electroactive materials that are different than that of
lithium metal, such as carbon intercalation materials, alloys
(e.g., of silicon) and combinations thereof such as carbon silicon
composites. For example, one method in accordance with the present
invention involves: i) providing a carbon anode in the fully
discharged state (i.e., entirely un-intercalated); ii) providing an
aqueous sulfur catholyte comprising water and dissolved lithium
sulfide; iii) providing a cathode comprising an electron transfer
medium for electrochemical oxidation of dissolved lithium sulfide;
iv) configuring the anode, catholyte and cathode into a battery
cell; and iv) charging the battery cell. Accordingly, in various
embodiments the instant cell comprises both dissolved lithium
sulfide and a significant amount of solid phase lithium sulfide in
contact with the aqueous electrolyte. For instance, in various
embodiments the molar quantity of active sulfur as solid phase
lithium sulfide is greater than that of active sulfur dissolved in
the electrolyte by a factor of at least 2, or at least 3, or at
least 5 or at least 10. Moreover, in the same or separate
embodiments, the full charge capacity of the cell just prior to
initial cell operation is derived from the ampere-hour capacity of
dissolved active sulfur species in the catholyte combined with the
ampere-hour capacity of solid phase lithium sulfide. Furthermore,
in the same or separate embodiments upon cell fabrication and just
prior to initial cell operation the anode electroactive material is
substantially devoid of active lithium, and the initial cell
operation is to charge the battery. For example, the anode
electroactive material may be an intercalation material capable of
electrochemically intercalating lithium upon electro-reduction in
the presence of lithium ions, or an alloying material capable of
electrochemically alloying with lithium upon electro-reduction in
the presence of lithium ions, or a material capable of forming a
lithium inter-metallic phase upon electro-reduction in the presence
of lithium ions. For example, in particular embodiments the anode
electroactive material is an intercalating carbon, silicon, or a
composite of said silicon and carbon.
In applications where high pulse power and size are paramount
performance benefit may be gained by taking advantage of the facile
electro-kinetics of solution phase redox in combination with the
high solubility of polysulfide species in water. For instance, in
various embodiments, the cell is formulated and operated such that
the ampere-hour capacity in the cell, at full state of charge, is
solely present as dissolved active sulfur species in the catholyte.
In particular the cell may be fabricated in the fully charged state
devoid of solid phase active sulfur (e.g., devoid of elemental
sulfur).
The use of water as a catholyte solvent clearly provides
considerable benefit, but it also presents significant challenges
in a lithium-sulfur battery. In particular, the use of water is
constrained by its reactivity with electroactive lithium materials
(e.g., lithium metal). Accordingly, the present invention makes use
of lithium anode structures wherein the electroactive lithium is
isolated from contacting the aqueous sulfur catholyte. In various
embodiments, a protected lithium electrode is employed which
contains a lithium electroactive material protected from the
external environment by a substantially impervious lithium ion
conductive protective membrane architecture. Thus in accordance
with the instant invention the aqueous catholyte is disposed in the
cell such that it directly contacts the electron transfer medium
but does not contact the electroactive material of the anode (e.g.,
lithium metal or carbon intercalation material).
A further challenge to the use of water in a lithium-sulfur cell is
the hydrolysis of dissolved lithium sulfide (Li.sub.2S) in the
catholyte and the resulting generation of hydrogen sulfide
(H.sub.2S). According to some embodiments of the present invention,
a lithium-sulfur cell can comprise a housing configured to contain
and withstand the pressure of such gas generation to maintain cell
integrity and safety. According to further embodiments, the pH of
the electrolyte (catholyte) can be adjusted to reduce or prevent
Li.sub.2S hydrolysis. This is particularly achieved with basic pHs,
for example greater than 7, or from about 9 to 12 and up to 14.
However, the invention is not limited to basic electrolytes, and it
is contemplated herein that the pH may be adjusted to values below
pH 7 (i.e., acidic) or about pH 7 (i.e., neutral catholyte) using
acidic salts and buffering agents.
Further relating to suitable electrolyte/catholyte formulations in
accordance with the present invention, compositions and methods are
provided to enhance contact between the aqueous electrolyte and the
cathode electron transfer medium, for example an electronically
conductive matrix such as a carbon or metal mesh, foam or other
high surface area, typically porous, structure. Such improved
contact enhances utlilization and rate performance of the cell.
Electrolyte/catholyte compositions in this regard can include a
surfactant to wet the catholyte to the conductive matrix. Also or
alternatively, the matrix can be surface treated prior to contact
with the electrolyte to enhance wetting, for example being soaked
in a wetting agent, followed by displacement of the wetting agent
with the aqueous catholyte solution of polysulfides. Still further
in this regard, the catholyte may include dissolved organosulfur as
a cathode active material. The organosulfur compound or compounds
can self-wet to the cathode electron transfer matrix
Another aspect of the present invention relates to the challenge
presented in an aqueous lithium-sulfur battery with regard to the
voltage stability window of water and the active sulfur (e.g.,
dissolved polysulfide) redox potentials. In order to expand the
redox potential window in which an aqueous lithium-sulfur battery
cell may operate without generating hydrogen and oxygen from the
water in the electrolyte, battery cells in accordance with
embodiments of the present invention may include a material with a
high overpotential for hydrogen (H.sub.2) and/or oxygen (O.sub.2)
in the cathode, in particular as or as part of the electron
transfer medium of the cathode. For example, a cathode matrix can
be formed from a metal with a high overpotential for H.sub.2, such
as lead (Pb). Or, a metal with a high overpotential for H.sub.2
(and/or O.sub.2) can be coated as an exterior layer on an
underlying matrix structure (also sometimes referred to herein as a
"core" or "core structure"). In some such embodiments, the
underlying matrix structure can be an electronic insulator (e.g., a
glass or polymer) so that discontinuities in the coating do not
result in the generation of hydrogen (or oxygen) gas at an
underlying conductor's surface. By providing a cathode electron
transfer medium with a high overpotential for H.sub.2 and/or
O.sub.2 battery cells in accordance with the present invention have
an extended operating potential range, beyond that of the potential
window of water.
Yet another aspect of the present invention relates to compositions
defining the exterior surface of the cathode electron transfer
medium (e.g., matrix) that electro-catalyze sulfur redox but also
have a high overpotential for H.sub.2, such as metal sulfides
(e.g., lead sulfide, cadmium sulfide, cobalt sulfide and nickel
sulfide) and in this way can provide both catalysis and high
overpotential for H.sub.2 as described above. Such coatings should
allow effective electron tunneling so as not to disrupt the
electron transfer function of the matrix. The coatings may be
applied to a conventional conductive matrix material, such as
carbon, or to a matrix material having a high overpotential for
H.sub.2, such as described above.
In yet another aspect the present invention relates to catholyte
formulations including the incorporation of one or more non-aqueous
solvents for particular benefit. Non-aqueous solvents suitable for
use herein to improve performance of the instant aqueous lithium
sulfur battery cells include aprotic and protic organic solvents
and ionic liquids.
In particular embodiments the aqueous catholyte comprises water and
a protic solvent that is non-aqueous, especially protic organic
solvents that are capable of dissolving a significant amount of
Li.sub.2S (e.g., methanol). Addition of the non-aqueous protic
solvent is particularly useful in cells that may be operated at
temperatures below the freezing temperature of water and yet still
require high solubility for lithium sulfide. Accordingly, in
various embodiments the catholyte is formulated with an amount of a
non-aqueous protic solvent (e.g., ethylene glycol) sufficient to
achieve a freezing point temperature (i.e., melt temperature) below
a desired value; for example, below -5.degree. C., -10.degree. C.,
-20.degree. C., -30.degree. C. or -40.degree. C.
While the invention has generally been described with reference to
an electroactive catholyte (i.e., a catholyte containing dissolved
active sulfur species) and/or electroactive fully reduced solid
phase lithium sulfide loaded in the cathode, the invention is not
limited as such, and it is contemplated that fully oxidized solid
phase electroactive sulfur (e.g., elemental sulfur) or active
organosulfur compounds may be incorporated in the cell during
fabrication as an exclusive source of active sulfur or in
combination with an electroactive sulfur catholyte. Notwithstanding
the aforementioned sulfur containing cathode configurations, in
various embodiments the cell is fabricated absent elemental sulfur,
and the cathode is, thereby, devoid of elemental sulfur just prior
to initial cell operation.
The invention also relates to methods of manufacture of aqueous
lithium-sulfur battery cells. In one aspect, such a method involves
de-oxygenating the catholyte and forming and sealing the cell in an
inert or reducing environment devoid of molecular oxygen (e.g., a
nitrogen environment) in order to reduce or eliminate free oxygen
(O.sub.2) in the catholyte solution. In this way the irreversible
oxidation of sulfur species in the aqueous catholyte (e.g.,
oxidation leading to insoluble thiosulfates) and the resultant loss
of active material, is reduced or avoided.
In various embodiments the instant cells are self-contained and
sealed in a hermetic casing wherein the entirety of the cell
capacity is derived from electroactive sulfur and electroactive
lithium disposed in the casing during cell manufacture. These fully
sealed cells may be of the primary or secondary type.
In other embodiments the instant cells are configured in a battery
flow cell system, wherein an aqueous sulfur catholyte is caused to
flow, and/or circulate, into the cell, and, in various embodiments,
through an inter-electrode region between the lithium anode and the
cathode electron transfer medium. In some embodiments both the
aqueous catholyte and the electroactive lithium are flowable and
during operation are caused to flow through the cell.
It should be understood that aqueous lithium-sulfur battery cells
in accordance with the present invention are not merely different
from conventional non-aqueous Li--S battery cells by their
substitution of a non-aqueous electrolyte solvent with an aqueous
electrolyte solvent system. The use of water in the electrolyte
results in a solvent system that is not just a spectator, but
actually participates in the electrochemical reactions at the
cathode, reacting to form and dissolve new species. The present
invention is therefore directed to an entirely new class of battery
cells having entirely different chemistry than conventional Li--S
battery cells (as evidenced by the dramatic difference in their
voltage profiles), and to the formulation, engineering, operation
and manufacturing challenges associated therewith.
These and other aspects of the present invention are described in
more detail, including with reference to figures, in the
description which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross section of a battery cell in accordance
with various embodiments of the present invention.
FIGS. 2A-B illustrates an electron transfer medium in accordance
with various embodiments of the present invention.
FIG. 3 is a qualitative illustration of a Pourbaix diagram for
water and active sulfur species in catholyte in accordance with the
present invention.
FIG. 4 is a photograph comparing the solubility of Li.sub.2S in
water with that in a non-aqueous solvent.
FIGS. 5A-D illustrate various alternative configurations of a
protective membrane architecture in accordance with the present
invention.
FIG. 6 is a schematic cross section of a battery flow cell system
in accordance with an embodiment of the present invention.
FIG. 7 is a schematic cross section of a battery flow cell system
in accordance with an alternative embodiment of the present
invention.
FIG. 8 is a plot comparing the cyclic voltammogram of an aqueous
lithium sulfur cell in accordance with an embodiment of the present
invention and a cell without active sulfur.
FIG. 9 is a cyclic voltammetric plot comparing the potential window
for aqueous lithium sulfur cell operation using two different
cathode materials.
FIG. 10 is a cyclic voltammetric plot comparing alternative aqueous
lithium sulfur cell embodiments in accordance with the present
invention.
FIG. 11 is a voltage vs. time cycling profile and a capacity vs.
cycle number profile for an aqueous lithium sulfur cell in
accordance with the present invention.
FIG. 12 is a voltage vs. capacity profile for an aqueous lithium
sulfur cell in accordance with the present invention.
FIG. 13 is a voltage vs. time cycling profile and a capacity vs.
cycle number profile for an aqueous lithium sulfur cell in
accordance with an embodiment of the present invention.
FIG. 14 is a voltage vs. time cycling profile and a capacity vs.
cycle number profile for an aqueous lithium sulfur cell in
accordance with an embodiment of the present invention.
FIG. 15 is a voltage vs. time cycling profile and a capacity vs.
cycle number profile for an aqueous lithium sulfur cell in
accordance with an embodiment of the present invention.
FIG. 16 is a voltage vs. time cycling profile for an aqueous
lithium sulfur cell in accordance with an embodiment of the present
invention.
FIG. 17 is a voltage vs. time cycling profile and a capacity vs.
cycle number profile for a lithium sulfur cell in accordance with
an embodiment of the present invention.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
Reference will now be made in detail to specific embodiments of the
invention. Examples of the specific embodiments are illustrated in
the accompanying drawings. While the invention will be described in
conjunction with these specific embodiments, it will be understood
that it is not intended to limit the invention to such specific
embodiments. On the contrary, it is intended to cover alternatives,
modifications, and equivalents as may be included within the spirit
and scope of the invention. In the following description, numerous
specific details are set forth in order to provide a thorough
understanding of the present invention. The present invention may
be practiced without some or all of these specific details. In
other instances, well known process operations have not been
described in detail so as to not unnecessarily obscure the present
invention.
A lithium sulfur cell in accordance with various embodiments of the
instant invention is shown in FIG. 1. The cell 100 includes a
cathode 110 comprising an electron transfer medium, a protected
lithium anode 120 and an aqueous electrolyte in contact with the
electron transfer medium and also in contact with an exterior
surface of the protected lithium anode.
The protected lithium anode 120 includes a lithium electroactive
material layer 122 and a substantially impervious lithium ion
conducting protective membrane architecture 126 on the surface of
the lithium active layer 122. The membrane architecture is
substantially impervious to water and has a first surface
chemically compatible in contact with the lithium electroactive
layer and a second surface, opposing the cathode, which is
chemically compatible in contact with water, and in particular
chemically compatible in contact with the catholyte employed in the
cell. In some embodiments the cell further includes a porous
separator material layer 130 interposed between the cathode and the
protected anode, and containing in its pores at least a portion of
the aqueous electrolyte (i.e., aqueous catholyte). In other
embodiments the cell is absent a separator and it is contemplated
herein that the membrane architecture second surface directly
contacts the cathode, which, in said embodiments, is generally
porous with catholyte filling the pore spaces.
The cathode 110 includes a solid electron transfer medium having an
"exterior surface" that is chemically compatible in contact with
the catholyte and on which dissolved active sulfur species are
electro-reduced during cell discharge and electro-oxidized on
charge. With reference to FIGS. 2A-B, in various embodiments the
electron transfer medium 200A/200B may be a porous
three-dimensional structure 200A or planar 200B and substantially
dense or otherwise porous (e.g., a planar mesh). Whether dense or
porous, the medium should be sufficiently electronically conductive
to support the electrical current through the cell and its exterior
surface capable of supporting the electron transfer current. When
porous, the solid electron transfer medium may take the form of a
porous matrix such as a woven or non-woven fiber network (e.g., a
metal or carbon fiber cloth or paper) or a through porous
monolithic solid body (e.g., a metal or carbon foam). When planar,
the medium may simply be a metal or carbonaceous sheet or foil or
open mesh of sufficient thickness and conductivity to be
self-supporting, or the planar medium may be a composite having a
first layer, typically thin and electronically conductive, that
defines the exterior surface and a second layer serving as a
substrate support, and optionally further providing current
collection when electronically conductive.
The electron transfer medium has an exterior surface that may be
porous or dense but is defined, at least in part, by a material
that, in contact with the catholyte, facilitates electron transfer,
and, in particular, facilitates electrochemical redox of active
sulfur species. Continuing with reference to FIGS. 2A-B, in various
embodiments the electron transfer medium 200A/200B is a porous
matrix composed of a core component (i.e., underlying matrix
structure) 210A/210B having an exterior layer component 220A/220B
that provides the exterior surface in contact with the catholyte.
The core component generally provides substrate support and may,
when conductive, facilitate current collection, whereas a primary
function of the exterior layer is to provide some benefit to the
electrochemical performance, and in particular that pertaining to
electron transfer. The exterior layer may be porous or dense. In
various embodiments, a dense exterior layer is also preferably
contiguous and therefore substantially covers the core surface in
its entirety. In other embodiments, a porous exterior layer is
suitable, especially when the surface composition of the core is
compatible with the catholyte and does not catalyze hydrogen
evolution, as described in more detail below. Furthermore, when
porous, the exterior layer may include high surface area particles
that electro-catalyze sulfur redox and/or increases the effective
surface area for electrical benefit.
In some embodiments the core, electronically conductive, supports
current collection, while the exterior layer primarily serves to
support and preferably enhance electrochemical sulfur redox. In
other embodiments the core is electronically insulating and the
exterior layer provides electron transfer and may provide some or
all of the current collector function. The insulating core may be
composed of any suitable insulating material of sufficient
mechanical integrity and is preferably although not necessarily
chemically compatible in contact with the catholyte. In certain
embodiments the exterior layer is dense and substantially free of
defects that otherwise would allow water from the electrolyte to
seep into contact with the core material, and potentially reduce
its strength or mechanical integrity. To prevent this from
happening, in preferred embodiments the core material is also
chemically compatible in contact with the catholyte and even more
preferably is a material that does not swell or lose mechanical
strength when in contact with water, and specifically does not
mechanically degrade or change shape if exposed to the active
electrolyte. In various embodiments additional layers may be
incorporated between the insulating or conductive core and the
exterior layer to support current collection and/or provide or
improve interface compatibility and/or adhesion. For example, the
insulating core of an underlying matrix structure may have a first
metal coating (e.g., aluminum) serving as an intermediary layer to
provide current collection and a second coating covering the
aluminum that defines, in whole or in part, the exterior surface
for the purpose of facilitating sulfur redox.
The electron transfer medium may be uncatalyzed, relying solely on
the medium material (e.g., carbon) to facilitate the
electrochemical redox reactions, or, in some embodiments, the
electron transfer medium may contain a catalyst on its surface,
such as a particulate catalyst or the catalyst may be formed on the
underlying carbon or metal matrix as a coating. In some embodiments
the exterior layer is a porous high surface area film composed of
electronically conductive particles (e.g., high surface area
carbons including nano-carbons, carbon blacks and functionalized
carbons) that preferably electro-catalyze at least one or both of
electro-reduction and electro-oxidation of active sulfur. In other
embodiments, as described in more detail below, the exterior layer
may be a dense, preferably thin, electronically conductive layer,
such as a thin dense film of a metal, metal alloy, or metal
compound (e.g., a metal sulfide) for the purposes of providing one
or more of electronic conduction, facilitation of sulfur redox, and
expansion of the voltage stability window of the catholyte, as
described in more detail below.
With regard to the voltage window of the catholyte, a significant
issue may arise during discharge once the cell voltage drops below
a "critical voltage" corresponding to the thermodynamic potential
for water reduction the cell electrochemistry is made complicated
by the potentiality of water decomposition, and in particular
H.sub.2 evolution. The issue is illustrated pictorially with
reference to FIG. 3, showing a Pourbaix diagram of water compared
to an illustrative Pourbaix diagram of sulfur redox without
assigning voltages to the sulfur electro-reduction/oxidation
reactions. As can be seen in the illustration, the critical voltage
varies with pH. For instance at pH 12 the critical voltage versus
lithium is about 2.3 Volts and decreases with increasing pH values,
reaching about 2.2 Volts at pH 14. As illustrated, albeit quite
qualitatively, at cell voltages below the voltage stability window
of water (i.e., below the critical voltage) there exist significant
active sulfur ampere-hour capacity; however, the practicality of
harnessing that capacity is complicated by water decomposition.
In this regard, the present invention provides cathode structures
having electron transfer mediums that enable the instant cells to
be discharged to voltages beyond the thermodynamic potential for
water reduction, and thereby efficiently harness the additional
ampere-hour capacity which exists at cell voltages below the
critical voltage, and preferably do so without evolving any
H.sub.2. Accordingly, in various embodiments, the exterior surface
of the electron transfer medium provides at least a dual
functionality: a first function to facilitate electrochemical
reduction/oxidation of the active sulfur species and a second
function to inhibit hydrogen evolution. For example, the exterior
surface may be defined in whole or in part by a material that
facilitates sulfur redox but has a high overpotential for H.sub.2
evolution. By this expedient the cell may be efficiently discharged
to voltages below the critical voltage without evolving H.sub.2.
Preferably the exterior surface has an overpotential of at least 50
mV beyond the thermodynamic potential of water reduction, and in
embodiments disclosed herein the overpotential is beyond 100 mV,
beyond 200 mV, beyond 300 mV, beyond 400 mV, beyond 500 mV, beyond
600 mV, and in certain embodiments beyond 700 mV and beyond 800 mV.
For instance, with regard to cell voltages, the use of high
overpotential electron transfer medium allows cells of the instant
invention to be discharged to cell voltages below 2.4 V, preferably
below 2.3 V, even more preferably below 2.2V, below 2.1V and yet
even more preferably below 2.0 V, below 1.9 V, below 1.8 V, below
1.7 V, below 1.6 V and below 1.5V.
Accordingly, in various embodiments at least a portion and in
certain embodiments the entirety of the exterior surface of the
electron transfer medium is defined by a material having a high
overpotential for H.sub.2 evolution. Suitable classes of such
materials include metals, metal alloys (e.g., amalgams), and metal
chalcogenides, especially metal sulfides. Particularly suitable
metals include lead, cadmium, indium, nickel, gallium, tellurium,
manganese, and zinc, or some combination thereof. Particularly
suitable metal alloys include amalgams. Particularly suitable metal
sulfides include cobalt sulfide, copper sulfide, nickel sulfide,
and zinc sulfide, or some combination thereof. The thickness of the
exterior layer is a tradeoff between burdening the cell with extra
weight and other considerations such as one or more of the
composition of the core material, mechanical strength, conductivity
and coating process. For instance, in embodiments the exterior
layer thickness may be in the range of 50 microns to values below 1
micron (e.g., about 0.5 microns or 0.25 microns). With regard to a
metal sulfide exterior layer the composition may vary gradually or
discretely across its thickness. For example, as described the
exterior layer may be formed in two steps, first the metal of the
metal sulfide may be coated, directly or indirectly, onto the core
component surface, and then the metal layer sulfidized to form a
thin layer of metal sulfide, which in embodiments may be thin and
dense, for example less than 10 nm, e.g., about 5 nm, about 2 nm or
about 1 nm. Such thin films are also self-healing in that if a
portion of the metal sulfide film were to flake off or start
cracking, the underlying metal layer surface would subsequently
react with sulfur in the catholyte to reform the sulfide film.
In a particular embodiment the porous electron transfer medium is
composed of a core component (e.g., a glass fiber mat) and a metal
sulfide exterior layer (e.g., cobalt sulfide or lead sulfide). The
core component may be electronically insulating, and the metal
sulfide formed by first applying a layer of the metal of the
sulfide on the core (e.g., coating the core with lead) and then
sulfidizing the metal coated core surface via treatment in a sulfur
containing environment. The metal layer may be applied using
coating methods applicable for both electronically conductive and
insulating core structures, as are known in the art generally,
including evaporation, dip coating from the melt,
electro-deposition and electroless deposition. Alternatively, the
core component may itself be composed of a material with a high
overpotential for H.sub.2 (e.g., a porous lead or porous cobalt
matrix). However, the use of a heavy metal core material may unduly
burden the overall cell weight, so in preferred embodiments the
core material is composed of a material of light weight and
preferably low density, such as carbon (e.g., graphitic like fibers
or carbon foams), light weight metals such as aluminum, or
inorganic materials such as silica or other glasses, or organic
materials such as polymers (e.g., polymer fibers). Hollow cores are
also contemplated herein for providing an exceptional lightweight
advantage. Carbon is a particularly useful core material as it can
be fabricated into a number of porous formats including porous
fiber matrices and foams, and is also electronically conductive and
thus capable of supporting current collection, which enables the
use of exceptionally thin exterior layers. For example, less than 5
micron thick, preferably less than 1 micron, and even more
preferably less than 0.5 micron, and yet even more preferably the
thickness of the exterior layer is less than 0.25 microns. In the
same or separate embodiments, especially when the core is
electronically insulating, an intermediate electronically
conductive layer (e.g., an aluminum layer) may be applied as a
coating between the core and the exterior layer to provide current
collection support or the exterior layer itself may be of
sufficient thickness to support the electrical current. For
instance an intermediate metal layer such as aluminum having
thickness between 0.25 microns and 10 microns, and more preferably
between 0.5 microns and 5 microns; for example, about 0.5 microns,
about 1 micron, about 2 microns, about 3 microns, about 4 microns,
and about 5 microns. Thereafter the exterior layer applied to the
surface of the intermediary layer using one or more of the
aforementioned coating techniques, or other coating techniques
generally known in the arts.
In various embodiments, the composition of the exterior surface may
be modified via surface treatments, and in particular,
sulfidization to form a sulfide composition suitable for
supporting, and preferably, electro-catalyzing sulfur redox. The
step of sulfidization may be carried out in-situ within the cell by
using a sulfur based catholyte. And while in-situ processing has
the clear advantage of simplicity, it also leads to a concomitant
loss in active sulfur cell capacity, since at least some of the
sulfur that would have otherwise provided cell capacity is consumed
by the sulfidization treatment, and for high surface area porous
matrix structures, the loss of active sulfur capacity can be
significant. Accordingly, in preferred embodiments for sulfidizing
porous matrix structures, the sulfidization step is carried out
ex-situ in a sulfur environment remote from the cell. For instance,
the core material composed of the metal of the metal sulfide, or a
core component coated with said metal may be placed in a bath of an
aqueous lithium polysulfide solution similar to or identical in
nature to the catholyte utilized in the cell, and allowed to stand
in the bath for a time sufficient to form a substantially dense and
pore free metal sulfide film.
Continuing with reference to FIG. 1 the cathode 110 may be
assembled in the cell devoid of elemental solid sulfur, and the
entirety of the sulfur capacity loaded into the cell via the
catholyte or solid phase Li.sub.2S. Alternatively, the cathode may
include some form of solid elemental sulfur, including crystalline
sulfur, amorphous sulfur, precipitated sulfur, and sulfur
solidified from the melt. Elemental sulfur includes the various
polyatomic molecules of sulfur, especially the octasulfur allotrope
characterized as cyclo-S.sub.8 ring, and polymorphs thereof such as
.alpha.-octasulfur, .beta.-octasulfur, and .gamma.-octasulfur. For
example, elemental sulfur (in the form of sulfur particulates
including nano-sized sulfur particles) may be incorporated in the
cell as a material component of the cathode, wherein, e.g., the
sulfur may be admixed with high surface area or activated carbon
particles and an appropriate binder (PTFE, PvDF and PEO) for
adhering the material components in a suitable liquid carrier for
formulating a slurry to be coated onto or impregnated into the
porous matrix structure. Slurry formulations, with or without solid
elemental sulfur, and coating methods suitable for use herein for
incorporating solid phase active sulfur into the cathode are
described in U.S. Pat. Nos.: 6,030,720, 6,200,704, and 6,991,662,
each of which is hereby fully incorporated by reference for all
that they describe, and in particular for the slurry formulations
and coating methods described. In the same or separate embodiments
the active sulfur in the cathode may be or further include
electroactive organosulfur compounds, including those described in
U.S. Pat. Nos.: 4,833,048; 4,917,974; 5,162,175; 5,516,598, hereby
fully incorporated by reference, in particular for their disclosure
relating to organosulfur compound composition and use.
In alternative embodiments, the cells may be assembled having all
of the sulfur capacity loaded in the cathode, e.g., in the form of
elemental sulfur. In other embodiments, sulfur is present in the
cathode as a solid phase electroactive material as well as in the
aqueous catholyte in the form of dissolved polysulfide species. In
some embodiments the cell is assembled using a cathode that is
loaded with solid phase Li.sub.2S, and by this expedient, the cell
may be assembled in the fully or partially discharged state,
wherein all or a portion of the active lithium is stored in or
nearby the cathode during cell assembly. The as assembled cell is
then subsequently charged, e.g., to full charge capacity, prior to
initial discharge.
Aqueous Sulfur Catholyte
In accordance with the instant invention, the aqueous catholyte
contains a significant amount of water (i.e., not merely a trace
amount), and the catholyte is disposed in the cell such that it
directly contacts the cathode. In certain embodiments water serves
as the main liquid solvent of the sulfur catholyte (i.e.,
electrolyte in contact with the sulfur cathode), and in particular
embodiments water is the only catholyte solvent.
In accordance with the instant invention a significant (non-trace)
amount of water is incorporated in the catholyte. In various
embodiments the volume percent of water in the catholyte relative
to the total liquid solvent volume is greater than 5%, 10%, 20%,
30%, 40%, 50%, 60%, 70%, 80%, and greater than 90%. In certain
embodiments water is the only liquid solvent in the catholyte, and
in particular embodiments thereof water is the only liquid solvent
(i.e., water constitutes 100% of the solvent volume of the
catholyte). In various embodiments water is the main solvent in the
catholyte.
Water has unique properties. In aqueous sulfur catholyte solutions,
water chemically interacts with the active sulfur species to
provide a number of benefits. In various embodiments the water
serves as a medium into which a large concentration of active
sulfur species may be dissolved (e.g., including sulfide anion
(S.sup.2-), polysulfide anion (S.sub.x.sup.2- with x>1),
hydrosulfide anion (HS.sup.-), polyhydrosulfide anion
(HS.sub.x.sup.- with x>1) and combinations thereof). In various
embodiments, the catholyte composition just prior to initially
operating the cell, which is typically the catholyte composition
upon cell fabrication and sealing, includes a significant
concentration of dissolved active sulfur species. For instance, an
active sulfur concentration in the catholyte of greater than 0.5
molar sulfur, greater than 1 molar sulfur, greater than 2 molar
sulfur, greater than 3 molar sulfur, greater than 4 molar sulfur,
greater than 5 molar sulfur, greater than 6 molar sulfur, greater
than 7 molar sulfur, greater than 8 molar sulfur, greater than 9
molar sulfur, greater than 10 molar sulfur, greater than 11 molar
sulfur, greater than 12 molar sulfur, greater than 13 molar sulfur,
greater than 14 molar sulfur, greater than 15 molar sulfur, greater
than 16 molar sulfur or greater than 17 molar sulfur may be
used.
Moreover, because it can be difficult to identify the precise
chemical nature of the various active sulfur species existing in
the catholyte solution at any given time during the course of
discharge or charge, the composition of the active species in the
catholyte is sometimes expressed herein, and in the claims, in
terms of an "active stoichiometric ratio" which is the ratio of
active sulfur to active lithium dissolved in the electrolyte, and
that ratio is represented by the general formula Li.sub.2S.sub.x.
Furthermore, it should be understood that the "active
stoichiometric ratio" as used herein is exclusive of any non-active
lithium salts and/or non-active sulfur salts that may be added to
the electrolyte for any purpose, including, e.g., to enhance
lithium ion conductivity in the case of e.g., a non-active LiCl
salt, or a non-active sulfur containing salt such as, e.g.,
LiSO.sub.3CF.sub.3.
Accordingly, in embodiments, the catholyte, just prior to initially
operating the cell, has an active stoichiometric ratio of
Li.sub.2S, Li.sub.2S.sub.x (x>1), Li.sub.2S.sub.x (1<x<5),
Li.sub.2S.sub.5, and Li.sub.2S.sub.x (x>5). For example, an
active stoichiometric ratio of about Li.sub.2S, about
Li.sub.2S.sub.2, about Li.sub.2S.sub.3, about Li.sub.2S.sub.4, and
about Li.sub.2S.sub.5.
In various embodiments, the lithium sulfur cells of the instant
invention include an aqueous catholyte having a high concentration
of dissolved active sulfur species. In embodiments, the sulfur
concentration of active sulfur species in the catholyte is greater
than 0.5 molar sulfur, greater than 1 molar sulfur, greater than 2
molar sulfur, greater than 3 molar sulfur, greater than 4 molar
sulfur, greater than 5 molar sulfur, greater than 6 molar sulfur,
greater than 7 molar sulfur, greater than 8 molar sulfur, greater
than 9 molar sulfur, greater than 10 molar sulfur, greater than 11
molar sulfur, greater than 12 molar sulfur, greater than 13 molar
sulfur, greater than 14 molar sulfur, greater than 15 molar sulfur,
greater than 16 molar sulfur or greater than 17 molar sulfur.
In particular embodiments, the active lithium sulfur stoichiometric
ratio in the catholyte just prior to initial cell operation is
Li.sub.2S; Li.sub.2S.sub.x (x>1); Li.sub.2S.sub.x (1<x<5);
Li.sub.2S.sub.5; and Li.sub.2S.sub.x (x>5), and the
concentration of the dissolved active sulfur species is typically
significant, e.g., greater than 1 molar sulfur. For instance, in
particular embodiments, especially for cells using a lithium metal
or lithium alloy as the electroactive anode material, the active
stoichiometric ratio just prior to initial cell operation is
Li.sub.2S.sub.x with the following range for x:
2.ltoreq.x.ltoreq.5, and the active sulfur concentration is between
10 to 17 molar sulfur. For example, a catholyte composition having
an active stoichiometric ratio of about Li.sub.2S.sub.4, and at
concentrations greater than 10 molar sulfur (e.g., 11, 12, 13, 14,
15, 16 or 17 molar sulfur) may be used. In another particular
embodiment, especially useful for cells which are fabricated in the
fully or mostly discharged state (e.g., having an anode
electroactive material that is devoid of active lithium), the
active stoichiometric ratio of the catholyte just prior to initial
cell operation is Li.sub.2S, and the active sulfur concentration is
typically greater than 1 molar sulfur, and preferably greater than
2 molar sulfur, and more preferably greater than 3 molar sulfur
(e.g., 3 molar, 4 molar, or 5 molar sulfur).
Of particular note is the high solubility and facile dissolution of
Li.sub.2S (lithium sulfide) in water. In non-aqueous aprotic
solvents lithium sulfide solubility is severely limited, and
Li.sub.2S is generally considered to be insoluble. Water is shown
herein to provide an excellent solvent for lithium sulfide
(Li.sub.2S), and this feature is used for advantage in various
embodiments of the instant invention in order to achieve high
ampere-hour (Ah) capacity per unit volume of catholyte, and
ultimately high cell energy density as well as improved
reversibility on deep discharge. A visual comparison is provided in
FIG. 5, illustrating that water has at least a 1000 fold greater
solubility for Li.sub.2S than that of tetraglyne (a common
non-aqueous solvent employed in conventional non-aqueous Li/S
cells).
Accordingly, in various embodiments the aqueous catholyte serves as
a medium into which high concentrations of Li.sub.2S dissolve.
Thus, by this expedient, aqueous lithium sulfur cells yielding a
high ampere-hour capacity per unit volume of catholyte can be
realized, and these high capacity cells may be deeply discharged
repeatedly since the reaction product (e.g., Li.sub.2S) is readily
dissolved and therefore more readily oxidized on charge. Thus, in
various embodiments, at the end of discharge a significant portion
of the sulfur ampere-hour capacity is present in the cell in the
form of solid phase discharge product (e.g., Li.sub.2S). For
instance, in embodiments, the end of discharge ratio comparing the
number of moles of sulfur as solid phase sulfur (e.g., Li.sub.2S)
to the number of moles of sulfur dissolved in the catholyte (e.g.,
as Li.sub.2S) is greater than 2; greater than 3; greater than 5, or
greater than 10.
Furthermore, the combination of high solubility and fast
dissolution kinetics of Li.sub.2S in water also enables a practical
method of making an aqueous lithium sulfur cell that is assembled
in the fully discharged state, and which makes use of alternative
lithium electroactive materials that are different than that of
lithium metal, such as carbon intercalation materials, alloys
(e.g., of silicon) and combinations thereof such as carbon silicon
composites. For example, one method in accordance with the present
invention involves: i) providing a carbon anode in the fully
discharged state (i.e., entirely un-intercalated); ii) providing an
aqueous polysulfide catholyte comprising water and dissolved
lithium sulfide; iii) providing a cathode comprising an electron
transfer medium for electrochemical oxidation of dissolved lithium
sulfide; iv) configuring the anode, catholyte and cathode into a
battery cell; and iv) charging the battery cell.
Whereas the fast dissolution kinetics of Li.sub.2S enables repeated
deep discharge, additional benefit may be gained by taking
advantage of the facile electro-kinetics of solution phase redox in
combination with the high solubility of polysulfide species in
water. Thus, in various embodiments, the cell is formulated such
that at full state of charge the catholyte contains a high
concentration of dissolved active sulfur species, and in certain
embodiments the cell is formulated and operated such that the
ampere-hour capacity of sulfur in the cell at full state of charge
is solely present as dissolved species in the catholyte.
Without intending to be limited by theory, lithium sulfide
dissolution in water involves hydrolysis that is believed to take
place in accordance with the following equilibrium:
S.sup.2-+HOH.rarw..fwdarw.HS.sup.-+OH.sup.-
Thus the pH of highly concentrated aqueous catholyte solutions of
Li.sub.2S dissolved in water is generally quite high and typically
greater than pH 10, and more typically greater than pH 11 or even
higher, e.g., about pH 12, about pH 13, or about pH 14. However,
the invention is not exclusively limited to cells having an aqueous
sulfur catholyte of such high pH, as the pH may be tailored using
pH adjusting additives, including basic salts (e.g., LiOH), acidic
salts (e.g., HCl) and buffering agents as are known to those of
skill in the art.
Thus, in various embodiments the catholyte may be formulated having
a pH that renders it acidic (i.e., pH<7), basic (i.e., pH>7),
or neutral (pH about 7).
The aqueous catholyte may further comprise a supporting lithium
salt to maintain a consistent and high conductivity over the entire
discharge and/or improve stability. Typically the supporting salt
concentration is in the range of 0.05 to 1.5 moles/liter (e.g.,
about 0.25 moles/liter). Examples of suitable supporting salts
include a variety of lithium cation salts. For instance, lithium
halides (e.g., LiCl, LiBr), LiSO.sub.3CF.sub.3,
LiN(CF.sub.3SO.sub.2).sub.2 and LiN(SO.sub.2C.sub.2F.sub.5).sub.2.
Typically present in the catholyte to a concentration of about 0.05
to 1.5 molar lithium, e.g., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8,
0.9, or 1.0 molar lithium.
Electroactive aqueous catholytes in accordance with the instant
invention comprise water and an active sulfur species dissolved
therein. In various embodiments the active sulfur species are
formed in the catholyte by reacting one or more precursor materials
with each other and/or with water. In one embodiment a first
precursor of lithium sulfide and a second precursor of elemental
sulfur are reacted in stoichiometric amounts in the presence of
water to yield active sulfur species in solution. Preferably, to
mitigate the undesirable formation of insoluble products of
oxidation (e.g., thiosulfates), the water should be deoxygenated
(i.e., the water should be substantially devoid of molecular
oxygen), which may be carried out by any suitable method known in
the art, including boiling of the water and/or purging the water
with an oxygen free gas, such as nitrogen. The purging step
continued until the desired level of oxygen has been reached. For
instance, the concentration of molecular oxygen in the catholyte is
preferably less than 1000 ppm, and more preferably less than 500
ppm and even more preferably less than 100 ppm, or less than 50 ppm
or even 10 ppm.
In various embodiments the aqueous catholyte further comprises one
or more non-aqueous solvents. In various embodiments the volume
percent of non-aqueous solvents in the catholyte ranges from about
1% to as much as 90% by volume; for example, between 1% and 10%,
between 10% and 20%, between 20% and 30%, between 30% and 40%,
between 40% and 50%, between 50% and 60%, between 60% and 70%,
between 70% and 80%, between 80% and 90%.
Non-aqueous solvents suitable for use herein to improve performance
include aprotic and protic organic solvents (solids and liquids,
typically liquids or solid polyethylene oxide) and ionic liquids.
In particular, in some embodiments protic organic solvents may be
used.
Examples of suitable non-aqueous aprotic and protic solvents
include ethers (e.g., 2-Methyltetrahydrofuran (2-MeTHF),
Tetrahydrofuran (THF), 4-Methyldioxolane (4-MeDIOX),
Tetrahydropyran (THP) and 1,3-Dioxolane (DIOX)) glymes (e.g.,
1,2-dimethoxyethane (DME/mono-glyme), di-glyme, tri-glyme,
tetra-glyme and higher glymes), carbonates (e.g., cyclic carbonates
such as propylene carbonate (PC), ethylene carbonate (EC), acyclic
carbonates such as dimethyl carbonate (DMC), ethylmethyl carbonate
(EMC) and diethyl carbonate (DEC), formates (e.g., Methyl Formate)
and butyrolactone (GBL). Other suitable aprotic solvents include
those having a high donor number (i.e., donor solvents) such as
hexamethylphosphoramide, pyridine, N,N-diethylacetamide (DMAC),
N,N-diethylformamide, dimethylsulfoxide (DMSO), tetramethylurea
(TMU), N,N-dimethylacetamide, N,N-dimethylformamide (DMF),
tributylphosphate, trimethylphosphate,
N,N,N',N'-tetraethylsulfamide, tetraethylenediamine,
tetramethylpropylenediamine, and pentamethyldiethylenetriamine.
Preferred donor solvents have a donor number of at least 15, more
preferably between about 15 and 40 and most preferably between
about 18-40. Particularly preferred donor solvents include
N,N-diethylformamide, N,N-dimethylformamide (DMF),
dimethylsulfoxide (DMSO), N,N-dimethylacetamide (DMAC); for
example, DMF. Suitable acceptor solvents which can be characterized
as Lewis acids (they may be protic or aprotic solvents) and promote
solvation of anions. Examples include alcohols such as methanol,
glycols such as ethylene glycol and polyglycols such as
polyethylene glycol as well as nitromethane, triflouroacetic acide,
trifluoromethanesulfonic acid, sulfur dioxide and boron
triflouride, and ethylene glycol (EG). Others include nitriles
(e.g., acetonitrile (AN), higher nitriles, propionitrile,
succinonitrile, butyronitrile, benzonitrile), amides (e.g.,
formamide, N-methylformamide, N,N-dimethylformamide,
N,N-diethylformamide, (DMF), acetamide, N-methylacetamide,
N,N-dimethylacetamide (DMAC), N,N-diethylacetamide,
N,N,N'N'tetraethylsulfamide, tetramethylurea (TMU), 2-pyrrolidone,
N-methylpyrrolidone, N-methylpyrrolidinone), amines (e.g.,
butylamine, ehtylenediamine, triethylamine, pyridine,
1,1,3,3-tetramethylguanidine (TMG), tetraethylenediamine,
tetramethylpropylenediamine, pentamethyldiethylenetriamine,
organosulfur solvents (e.g., dimethylsulfoxide (DMSO), sulfolane,
other sulfones, dimethylsulfite, ethylene sulfite, and
organophosphorous solvents (e.g., tributylphosphate,
trimethylphosphate, hexamethylphosphoramide (HMPA)).
In the same or separate embodiments a non-aqueous solvent may be
added to the aqueous catholyte to effect dissolution of elemental
sulfur. The addition of such a solvent (e.g., toluene or carbon
disulfide, preferably toluene) can enable charging to elemental
sulfur (dissolved or precipitated).
While the use of non-aqueous solvents such as aprotic organic
solvents, typically liquids, but not limited as such, may be useful
for facilitating the dissolution of high order polysulfide species,
protic solvents and ionic liquids may also be incorporated in the
aqueous catholyte to further enhance dissolution of lithium sulfide
or more generally improve cell performance.
For instance, in particular embodiments the aqueous catholyte
comprises water and a protic solvent that is non-aqueous,
especially protic organic solvents that are capable of dissolving a
significant amount of Li.sub.2S. Particularly suitable non-aqueous
protic solvents are organic solvents such as alcohols, diols,
triols and polyols, especially alcohols (e.g., methanol and
ethanol) and diols (e.g., ethylene glycol). Addition of the
non-aqueous protic solvent is particularly useful in cells that may
be operated at temperatures below the freezing temperature of water
and yet still require high solubility for lithium sulfide.
Accordingly, in various embodiments the catholyte is formulated
with an amount of a non-aqueous protic solvent to achieve a desired
freezing point temperature (i.e., melt temperature), including
formulations wherein the melt temperature is less than 0.degree.
C., less than -5.degree. C., less than -10.degree. C., less than
-15.degree. C., less than -20.degree. C., less than -30.degree. C.,
and less than -40.degree. C. Moreover, it is contemplated herein
that the non-aqueous protic solvent may be present in high
concentration in the catholyte, including 10%-20%, 20%-30%,
30%-40%, 40%-50%, 50%-60%, 60%-70%, 70%-80%, 80%-90% (e.g., such
said volume percents of methanol, ethanol or ethylene glycol or
combinations thereof.
Contact between the aqueous electrolyte and the cathode electron
transfer medium, for example an electronically conductive matrix
such as a carbon or metal mesh, foam or other high surface area,
typically porous, structure, may be enhanced by electrolyte
additives and/or co-solvents. Such improved contact enhances
utlilization and rate performance of the cell.
Electrolyte/catholyte compositions in this regard can include a
surfactant, such as a polyol or polyglycol, for example PEG, to wet
the catholyte to the conductive matrix. Also or alternatively, the
matrix can be surface treated prior to contact with the electrolyte
to enhance wetting, for example being soaked in a wetting agent,
such as methanol or ethylene glycol, followed by displacement of
the wetting agent with the aqueous catholyte solution of
polysulfides. Still further in this regard, the catholyte may
include dissolved organosulfur as a cathode active material. The
organosulfur compound or compounds can self-wet to the cathode
electron transfer matrix.
Lithium Anode
Typically, when using a protected lithium electrode as described
below in which a solid electrolyte membrane provides isolation of
the electroactive material against contact with the aqueous
catholyte, the catholyte is devoid of certain extraneous ions which
would otherwise interfere with the cell functionality, including
contaminating the membrane via diffusion into the conductive
atomically formed channels. Accordingly, in various embodiments of
the instant invention the aqueous catholyte is substantially devoid
of alkali metal cations other than lithium, and more preferably
substantially devoid of all metal cations other than lithium. For
example the catholyte is devoid of sodium and potassium ions, which
is to mean that there is substantially no sodium or potassium ions
in the electrolyte.
The cell comprises a Li anode. The lithium electroactive material
of the anode is typically in layered form and may be Li metal or a
Li metal alloy (e.g., silicon) or Li intercalation material (e.g.,
lithiated carbon) or in a particular embodiment a silicon carbon
composite. In one example, a Li metal foil may be used. In another
example lithium ion anodes, which are well known in the battery
art, are used as the electroactive lithium material layer (e.g., a
carbon intercalation material coated on a copper current
collector). Electroactive lithium materials, including
intercalation host compounds and lithium alloys and lithium metal
are well known in the lithium battery art. In certain embodiments
the anode is lithium metal (e.g., in foil or sintered form) and of
sufficient thickness (i.e., capacity) to enable the cell to achieve
the rated discharge capacity of the cell. The anode may take on any
suitable form or construct including a green or sintered compact
(such as a wafer or pellet), a sheet, film, or foil, and the anode
may be porous or dense. Without limitation, the lithium anode may
have a current collector (e.g., copper foil, or suitable expandable
metal) pressed or otherwise attached to it in order to enhance the
passage of electrons between it and the leads of the cell. Without
limitation the cell may be anode or cathode limited. When anode
limited, the complete discharge (corresponding to rated capacity)
will substantially exhaust all the lithium in the anode. When
cathode limited, some active lithium will remain subsequent to the
cell delivering its rated capacity.
The anode is protected with a protective membrane architecture
chemically stable to both the anode and the environment of the
adjacent sulfur cathode. The protective membrane architecture
typically comprises a solid electrolyte protective membrane and an
interlayer. The solid electrolyte protective membrane is sometimes
referred to herein as ion membrane. The protective membrane
architecture is in ionic continuity with the Li anode and is
configured to selectively transport Li ions while providing an
impervious barrier to the environment external to the anode.
Protective membrane architectures suitable for use in the present
invention are described in applicants' U.S. Pat. Nos. 7,645,543;
7,666,233; 8,048,571; and 7,282,295, incorporated by reference
herein in their entirely, and in particular for their description
of protective membrane structures and architectures.
FIGS. 5A-D illustrate representative protective membrane
architectures from these disclosures suitable for use in the
present invention. The protective membrane architectures provide a
barrier to isolate a Li anode from ambient and/or the cathode side
of the cell while allowing for efficient ion Li metal ion transport
into and out of the anode. The architecture may take on several
forms. Generally it comprises a solid electrolyte layer that is
substantially impervious, ionically conductive and chemically
compatible with the external ambient (e.g., air or water) or the
cathode environment.
Referring to FIG. 5A, the protective membrane architecture can be a
monolithic solid electrolyte 502 that provides ionic transport and
is chemically stable to both the active metal anode 501 and the
external environment. Examples of such materials are LiHfPO.sub.4,
LISICON (the lithium-stable analog to NASICON),
Li.sub.5La.sub.3Ta.sub.2O.sub.12 and
Li.sub.5La.sub.3Nb.sub.2O.sub.12, Na.sub.5MSi.sub.4O.sub.12 (M:
rare earth such as Nd, Dy, Gd) and the garnet-like structures
described below.
More commonly, the ion membrane architecture is a composite
composed of at least two components of different materials having
different chemical compatibility requirements, one chemically
compatible with the anode, the other chemically compatible with the
exterior; generally ambient air or water, and/or battery
electrolytes/catholytes. By "chemical compatibility" (or
"chemically compatible") it is meant that the referenced material
does not react to form a product that is deleterious to battery
cell operation when contacted with one or more other referenced
battery cell components or manufacturing, handling, storage or
external environmental conditions. The properties of different
ionic conductors are combined in a composite material that has the
desired properties of high overall ionic conductivity and chemical
stability towards the anode, the cathode and ambient conditions
encountered in battery manufacturing. The composite is capable of
protecting an active metal anode from deleterious reaction with
other battery components or ambient conditions while providing a
high level of ionic conductivity to facilitate manufacture and/or
enhance performance of a battery cell in which the composite is
incorporated.
Referring to FIG. 5B, the protective membrane architecture can be a
composite solid electrolyte 510 composed of discrete layers,
whereby the first material layer 512 (also sometimes referred to
herein as "interlayer") is stable to the active metal anode 501 and
the second material layer 514 is stable to the external
environment. Alternatively, referring to FIG. 5C, the protective
membrane architecture can be a composite solid electrolyte 520
composed of the same materials, but with a graded transition
between the materials rather than discrete layers.
Generally, the solid state composite protective membrane
architectures (described with reference to FIGS. 5B and C have a
first and second material layer. The first material layer (or first
layer material) of the composite is ionically conductive, and
chemically compatible with an active metal electrode material.
Chemical compatibility in this aspect of the invention refers both
to a material that is chemically stable and therefore substantially
unreactive when contacted with an active metal electrode material.
It may also refer to a material that is chemically stable with air,
to facilitate storage and handling, and reactive when contacted
with an active metal electrode material to produce a product
in-situ that is chemically stable against the active metal
electrode material and has the desirable ionic conductivity (i.e.,
a first layer material). Such a reactive material is sometimes
referred to as a "precursor" material. The second material layer of
the composite is substantially impervious, ionically conductive and
chemically compatible with the first material. Additional layers
are possible to achieve these aims, or otherwise enhance electrode
stability or performance. All layers of the composite have high
ionic conductivity, at least 10.sup.-7S/cm, generally at least
10.sup.-6S/cm, for example at least 10.sup.-5S/cm to 10.sup.-4S/cm,
and as high as 10.sup.- .sup.3S/cm or higher so that the overall
ionic conductivity of the multi-layer protective structure is at
least 10.sup.-7S/cm and as high as 10.sup.-3S/cm or higher.
A fourth suitable protective membrane architecture is illustrated
in FIG. 5D. This architecture is a composite 530 composed of an
interlayer 532 between the solid electrolyte 534 and the active
metal anode 501 whereby the interlayer is includes a non-aqueous
liquid, gel or solid polymer electrolyte polymer phase anolyte.
Thus, the architecture includes an active metal ion conducting
separator layer with a non-aqueous anolyte (i.e., electrolyte in
contact with the anode electroactive material), the separator layer
being chemically compatible with the active metal and in contact
with the anode; and a solid electrolyte layer that is substantially
impervious (pinhole- and crack-free) ionically conductive layer
chemically compatible with the separator layer and aqueous
environments and in contact with the separator layer. The solid
electrolyte layer of this architecture (FIG. 5D) generally shares
the properties of the second material layer for the composite solid
state architectures (FIGS. 5B and C). Accordingly, the solid
electrolyte layer of all three of these architectures will be
referred to below as a second material layer or second layer.
A wide variety of materials may be used in fabricating protective
composites in accordance with the present invention, consistent
with the principles described above. For example, in the solid
state embodiments of FIGS. 5B and 5C, the first layer (material
component), in contact with the active metal, may be composed, in
whole or in part, of active metal nitrides, active metal
phosphides, active metal halides active metal sulfides, active
metal phosphorous sulfides, or active metal phosphorus
oxynitride-based glass. Specific examples include Li.sub.3N,
Li.sub.3P, LiI, LiBr, LiCl, LiF, Li.sub.2S--P.sub.2S.sub.5,
Li.sub.2S--P.sub.2S.sub.5--LiI and LiPON. Active metal electrode
materials (e.g., lithium) may be applied to these materials, or
they may be formed as reaction products in situ by contacting
precursors such as metal nitrides, metal phosphides, metal halides,
red phosphorus, iodine, nitrogen or phosphorus containing organics
and polymers, and the like with lithium. A particularly suitable
precursor material is copper nitride (e.g., Cu.sub.3N). The in situ
formation of the first layer may result from an incomplete
conversion of the precursors to their lithiated analog.
Nevertheless, such composite reaction products formed by incomplete
conversions meet the requirements of a first layer material for a
protective composite in accordance with the present invention and
are therefore within the scope of the invention.
For the anolyte interlayer composite protective architecture
embodiment (FIG. 5D), the protective membrane architecture has an
active metal ion conducting separator layer chemically compatible
with the active metal of the anode and in contact with the anode,
the separator layer comprising a non-aqueous anolyte, and a
substantially impervious, ionically conductive layer ("second"
layer) in contact with the separator layer, and chemically
compatible with the separator layer and with the exterior of the
anode. The separator layer can be composed of a semi-permeable
membrane impregnated with an organic anolyte. For example, the
semi-permeable membrane may be a micro-porous polymer, such as are
available from Celgard, Inc. The organic anolyte may be in the
liquid or gel phase. For example, the anolyte may include a solvent
selected from the group consisting of organic carbonates, ethers,
lactones, sulfones, etc, and combinations thereof, such as EC, PC,
DEC, DMC, EMC, 1,2-DME or higher glymes, THF, 2MeTHF, sulfolane,
and combinations thereof. 1,3-dioxolane may also be used as an
anolyte solvent, particularly but not necessarily when used to
enhance the safety of a cell incorporating the structure. When the
anolyte is in the gel phase, gelling agents such as polyvinylidine
fluoride (PVdF) compounds, hexafluropropylene-vinylidene fluoride
copolymers (PVdf-HFP), polyacrylonitrile compounds, cross-linked
polyether compounds, polyalkylene oxide compounds, polyethylene
oxide compounds, and combinations and the like may be added to gel
the solvents. Suitable anolytes will, of course, also include
active metal salts, such as, in the case of lithium, for example,
LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, LiSO.sub.3CF.sub.3 or
LiN(SO.sub.2C.sub.2F.sub.5).sub.2. One example of a suitable
separator layer is 1 M LiPF.sub.6 dissolved in propylene carbonate
and impregnated in a Celgard microporous polymer membrane.
The second layer (material component) of the protective composite
may be composed of a material that is substantially impervious,
ionically conductive and chemically compatible with the first
material or precursor, including glassy or amorphous metal ion
conductors, such as a phosphorus-based glass, oxide-based glass,
phosphorus-oxynitride-based glass, sulfur-based glass,
oxide/sulfide based glass, selenide based glass, gallium based
glass, germanium-based glass, Nasiglass; ceramic active metal ion
conductors, such as lithium beta-alumina, sodium beta-alumina, Li
superionic conductor (LISICON), and the like; or glass-ceramic
active metal ion conductors. Specific examples include LiPON,
Li.sub.3PO.sub.4.Li.sub.2S.Si.sub.2,
Li.sub.2S.GeS.sub.2.Ga.sub.2S.sub.3, Li.sub.2O.11Al.sub.2O.sub.3,
Na.sub.2O.11Al.sub.2O.sub.3,
Li.sub.1+xTi.sub.2-xAl.sub.x(PO.sub.4).sub.3
(0.1.ltoreq.x.ltoreq.0.9) and crystallographically related
structures, Li.sub.1+xHf.sub.2-xAl.sub.x(PO.sub.4).sub.3
(0.1.ltoreq.x.ltoreq.0.9), Li.sub.3Zr.sub.2Si.sub.2PO.sub.12,
Na.sub.5ZrP.sub.3O.sub.12, Li-Silicates,
Li.sub.0.3La.sub.0.5TiO.sub.3, Li.sub.5MSi.sub.4O.sub.12 (M: rare
earth such as Nd, Gd, Dy) Li.sub.5ZrP.sub.3O.sub.12,
Li.sub.5TiP.sub.3O.sub.12, Li.sub.3Fe.sub.2P.sub.3O.sub.12 and
Li.sub.4NbP.sub.3O.sub.12, and combinations thereof, optionally
sintered or melted. Suitable ceramic ion active metal ion
conductors are described, for example, in U.S. Pat. No. 4,985,317
to Adachi et al., incorporated by reference herein in its entirety
and for all purposes.
A particularly suitable glass-ceramic material for the second layer
of the protective composite is a lithium ion conductive
glass-ceramic having the following composition:
TABLE-US-00001 Composition mol % P.sub.2O.sub.5 26-55% SiO.sub.2
0-15% GeO.sub.2 + TiO.sub.2 25-50% in which GeO.sub.2 0-50%
TiO.sub.2 0-50% ZrO.sub.2 0-10% M.sub.2O.sub.3 0-10%
Al.sub.2O.sub.3 0-15% Ga.sub.2O.sub.3 0-15% Li.sub.2O 3-25%
and containing a predominant crystalline phase composed of
Li.sub.1+x(M,Al,Ga).sub.x(Ge.sub.1-yTi.sub.y).sub.2-x(PO.sub.4).sub.3
where X.ltoreq.0.8 and 0.ltoreq.Y.ltoreq.1.0, and where M is an
element selected from the group consisting of Nd, Sm, Eu, Gd, Tb,
Dy, Ho, Er, Tm and Yb and/or
Li.sub.1+x+yQ.sub.xTi.sub.2-xSi.sub.yP.sub.3-yO12 where
0<X.ltoreq.0.4 and 0.ltoreq.Y.ltoreq.0.6, and where Q is Al or
Ga. The glass-ceramics are obtained by melting raw materials to a
melt, casting the melt to a glass and subjecting the glass to a
heat treatment. Such materials are available from OHARA
Corporation, Japan and are further described in U.S. Pat. Nos.
5,702,995, 6,030,909, 6,315,881 and 6,485,622, incorporated herein
by reference.
Another particularly suitable material for the second layer of the
protective composite is a lithium ion conducting oxide having a
garnet like structure. These include
Li.sub.6BaLa.sub.2Ta.sub.2O.sub.12;
Li.sub.7La.sub.3Zr.sub.2O.sub.12, Li.sub.5La.sub.3Nb.sub.2O.sub.12,
Li.sub.5La.sub.3M.sub.2O.sub.12 (M=Nb,
Ta)Li.sub.7+xA.sub.xLa.sub.3-xZr.sub.2O.sub.12 where A may be Zn.
These materials and methods for making them are described in U.S.
Patent Application Pub. No.: 2007/0148533 (application Ser. No:
10/591,714), hereby incorporated by reference in its entirety, and
suitable garnet like structures are also described in International
Patent Application Pub. No.: WO/2009/003695 which is hereby
incorporated by reference for all that it contains, and in
particular for its description of garnet-like structures.
The composite should have an inherently high ionic conductivity. In
general, the ionic conductivity of the composite is at least
10.sup.-7 S/cm, generally at least about 10.sup.-6 to 10.sup.-5
S/cm, and may be as high as 10.sup.-4 to 10.sup.-3 S/cm or higher.
The thickness of the first precursor material layer should be
enough to prevent contact between the second material layer and
adjacent materials or layers, in particular, the active metal of
the anode. For example, the first material layer for the solid
state membranes can have a thickness of about 0.1 to 5 microns; 0.2
to 1 micron; or about 0.25 micron. Suitable thickness for the
anolyte interlayer of the fourth embodiment range from 5 microns to
50 microns, for example a typical thickness of Celgard is 25
microns.
The thickness of the second material layer is preferably about 0.1
to 1000 microns, or, where the ionic conductivity of the second
material layer is about 10.sup.-7 S/cm, about 0.25 to 1 micron, or,
where the ionic conductivity of the second material layer is
between about 10.sup.-4 about 10.sup.-3 S/cm, about 10 to 1000
microns, preferably between 1 and 500 microns, and more preferably
between 10 and 100 microns, for example about 20 microns.
Seals and methods of making seals which are particularly suitable
for sealing protected anodes described hereinabove and elsewhere,
including compliant and rigid seals, are fully described in U.S.
Patent Publication No.: 2007/0037058 and U.S. Patent Publication
No.: U.S. 2007/0051620 to Visco et al., and are hereby incorporated
by reference in their entirety, and in particular for their
descriptions of cell seals and sealing techniques.
Optional Separator
With reference to FIG. 1 an optional separator component 130 may be
interposed between the membrane architecture and the sulfur
cathode. Various separator materials suitable for use herein are
known in the battery arts. These separators include porous
inorganic mats, microporous polymer sheets, and gels. In a
particular embodiment the separator is a hydrogel comprising water
impregnated a polymer. In some embodiments the polymer itself may
also serve as a solid solvent for the dissolution of active sulfur
species, such as PEO and polyalcohols (e.g., polyvinyl
alcohol).
In various embodiments the instant battery cell is fabricated such
that the entirety of the cathode capacity is loaded into the cell
upon fabrication as dissolved polysulfide species (e.g., the active
stoichiometric ratio of Li.sub.2Sx with x is >1 e.g., about
Li.sub.2S.sub.2, about Li.sub.2S.sub.3, about Li.sub.2S.sub.4, and
about Li.sub.2S.sub.5). In certain embodiments solid phase sulfur
is added to further enhance cell capacity (i.e., the cathode active
species derived from a combination of dissolved polysulfide species
and solid elemental sulfur. In some embodiments the entirety of the
cathode active sulfur is loaded into the cathode as solid elemental
sulfur. While in other embodiments, as described herein, the
catholyte is in a fully reduced state composed of Li.sub.2S
dissolved in water, and in some embodiments thereof solid phase
Li.sub.2S may be dispersed in the catholyte or present as a solid
particle in the pores of the cathode or separator.
In accordance with various embodiments of the instant invention a
significant amount of the cathode ampere-hour capacity is derived
from the active aqueous sulfur catholyte, and that amount is
typically greater than 10%; for instance, greater than 20%, greater
than 30%, greater than 40%, greater than 50%, greater than 60%,
greater than 70%, greater than 80%, greater than 90%, and in
certain embodiments 100%.
Flow Cells
With reference to FIG. 6 there is illustrated a representative
embodiment of an aqueous lithium sulfur flow cell battery system
600 in accordance with the instant invention. The system includes a
reactor cell 660 in which there is positioned a lithium anode 120
and a sulfur cathode 110 configured, in one embodiment, in a
spatially apart relationship, therewith defining an inter-electrode
region 650 through which an aqueous sulfur catholyte is caused to
flow during operation. In various embodiments the lithium anode is
a protected lithium electrode as described above and the sulfur
cathode likewise as described above. In a slightly modified
embodiment the sulfur cathode, a porous three dimensional body, is
positioned in direct contact with the first surface of the
protected anode solid electrolyte membrane architecture (i.e., not
in a spatially apart relationship) and the aqueous catholyte is
caused to flow into the pores of the cathode structure.
Continuing with reference to FIG. 6 the system further comprises an
external reservoir system, which may take the form of a storage
tank 620 for storing the aqueous sulfur catholyte to be flowed
through the inter-electrode region or channel. The reservoir system
may also include pipeworks 610 for fluidly coupling the tank to the
reactor, and a pump 603 for circulating the electrolyte through the
channel. The pipeworks may have valves (not shown) for closing or
opening the reactor cell to the storage tank. The pump may be
operated for circulating the electrolyte through the channel, and
the valves may be used to control the flow of catholyte through the
reactor.
The aqueous catholyte provides the electroactive sulfur species,
which are electrochemically reacted at the sulfur electrode during
charge and discharge. In operation, the aqueous catholyte from the
storage tank is caused to flow by or through the sulfur cathode,
and dissolved polysulfide species are electro-reduced when the
system is delivering electricity (during discharge) and
electro-oxidized when storing electricity on charge.
Since the ampere-hour capacity of the cathode is provided by the
aqueous catholyte in the storage tank, the sulfur cathode is
typically assembled in the reactor cell devoid of elemental sulfur.
For instance, the sulfur cathode may be a carbon matrix optionally
coated with a catalyst to facilitate polysulfide redox while
inhibiting hydrogen evolution. Moreover, during system assembly,
while the lithium electroactive material of the anode may be
incorporated in a fully charged state (e.g., in the form of a
lithium metal foil), in preferred embodiments it is an
intercalation material or alloy material that is incorporated in
the fully discharged state (i.e., devoid of any active lithium).
Carbon materials such as graphitic carbons capable of reversibly
intercalating lithium are a particularly suitable lithium
electroactive material for use in the instant flow cell system.
Others include lithium alloying materials, as described above, such
as silicon and tin which are capable of reversibly
absorbing/desorbing lithium electrochemically, as well as composite
carbon silicon materials.
Held in the storage tank, the aqueous catholyte effectively
provides the cathode fuel for the electrochemical reaction at the
sulfur cathode, and the aqueous catholytes embodiments described
above with reference to the battery cell embodiment illustrated in
FIG. 1 are suitable for use herein as a cathode fuel. The aqueous
catholyte fuel comprises polysulfide species dissolved in water. In
embodiments the concentration of the dissolved polysulfide species
in the electrolyte is in the range of 0.5 to 1 molar sulfur, 1 to 2
molar sulfur, 2 to 3 molar sulfur, 3 to 4 molar sulfur, 4 to 5
molar sulfur, 5 to 6 molar sulfur, 6 to 7 molar sulfur, 7 to 8
molar sulfur, 8 to 9 molar sulfur, 9 to 10 molar sulfur, and in
some embodiments the concentration of polysulfide species is
greater than 10 molar sulfur, greater than 11 molar, greater than
12 molar, greater than 13 molar, greater than 14 molar, greater
than 15 molar, and greater than 16 molar.
In one embodiment the system is assembled with the lithium
electroactive material in the discharged state (e.g., carbon
intercalation material devoid of intercalated lithium), and the
aqueous catholyte comprising highly reduced polysulfide species,
e.g., dissolved Li.sub.2S. For example, the aqueous catholyte a
solution of about 3 molar Li.sub.2S dissolved in water. Aqueous
sulfur catholyte storage tanks having enhanced sulfur capacity
(i.e., greater sulfur capacity per unit volume) may be achieved by
adding additional solid lithium sulfide to the catholyte beyond its
solubility limit (i.e., a saturated water solution of Li.sub.2S).
Because of the fast kinetics of lithium sulfide dissolution in
water, additional catholyte capacity may be added to the tank by
dispersing or suspending solid phase lithium sulfide in the aqueous
catholyte.
Continuing with reference to the above embodiment, the system is
assembled in the fully discharged state so it must undergo an
initial charge reaction to lithiate the carbon intercalation
material. The initial charge may be conducted via electro-oxidation
of the reduced aqueous catholyte (e.g., 3 molar Li.sub.2S water
solution) or a conditioning catholyte formulation may be used, for
instance one in which sulfur is not the electroactive species. For
example, the initial charge may be completed by using a water based
lithium nitrate catholyte solution that is circulated or caused to
flow past the cathode, whereupon the water is electro-oxidized and
oxygen evolved, while at the anode lithium ions from the
conditioning catholyte electro-reductively intercalate into the
carbon. The conditioning catholyte flowing through the channel may
be electro-oxidized until the reaction is complete and the carbon
is sufficiently or fully lithiated. Thereafter, the conditioning
catholyte tank is replaced by a tank of aqueous sulfur
catholyte.
In embodiments wherein the lithium electroactive material is fully
or mostly charged via the lithiation step described above (e.g., by
using a conditioning catholyte), the aqueous catholyte may then be
formulated in a highly oxidative state; for instance, as elemental
sulfur dispersed or suspended in a water solution typically also
comprising a dissolved lithium salt (e.g., lithium hydroxide) to
support the ionic current. It is contemplated that toluene may be
added to the catholyte in order to dissolved some of the dispersed
solid sulfur and by this expedient facilitate electro-reduction at
the sulfur cathode.
Various compositions of the as formulated catholyte storage tanks
are contemplated. In various embodiments the flow cell is operated
such that the active stoichiometric lithium sulfur ratio is
Li.sub.2S.sub.x with (1<x<5), (x=5), or (x>5)
In the aforementioned flow cell embodiments, the lithium
electroactive material is stationary, which is to mean that it is
non-flowing and incorporated as a component of the protected
lithium electrode, e.g., typically in the form of a layer such as a
sintered layer or a coating on a current collector as is well known
in the field of lithium ion batteries. Thus, the capacity of the
anode is set once the coating is formed and the system is
assembled.
In an alternative embodiment, with reference to the flow cell
system 700 illustrated in FIG. 7, the structure of FIG. 6 is
supplemented by a reactor cell 760 configured for through flow of a
flowable lithium electroactive material (e.g., an electroactive
lithium slurry) between an anode current collector 122 on which the
electrochemical reactions take place and the second surface of a
substantially impervious lithium ion conducting membrane
architecture 126. Flowable lithium electroactive materials suitable
for use herein are described in U.S. Patent Application Pub. Nos.:
2011/0200848 of Chiang et al., published Aug. 18, 2011 and
2010/0323264 of Chiang et al., published Dec. 23, 2011, and each of
these is hereby incorporated by reference for all that they contain
in this regard. Generally these are anode particles dispersed in an
ionically conductive carrier fluid that is compatible with the
anode particles over the range of oxidation state encompassing full
charge to full discharge. Particularly suitable anode particulates
are intercalation carbons or alloy materials such as silicon, or a
combination of these (e.g., carbon-silicon composite). The anode
current collector 122 is disposed in the cell in spaced relation to
the protective membrane architecture, thus defining a channel 702
through which the lithium electroactive slurry is caused to flow,
for instance via pumping action. The flow system includes a second
external reservoir system for the lithium anode, which may take the
form of a storage tank 720B for storing the lithium anode slurry
and pipeworks 710B for fluidly coupling the tank to the reactor
cell, and a pump 703B for circulating the slurry through the
channel, similar to that which is described above for circulating
the sulfur catholyte.
EXAMPLES
Aqueous Catholytes with Dissolved Active Sulfur Species
The following examples provide details illustrating the preparation
and advantageous properties, including high ionic conductivity, of
aqueous catholytes with dissolved active sulfur species suitable
for use in electrochemical cells in accordance with the present
invention. These examples are provided to exemplify and more
clearly illustrate aspects of the present invention and are in no
way intended to be limiting.
Example 1
This example pertains to the preparation and conductivity
measurement of a first active aqueous sulfur catholyte (i.e.,
Catholyte #1) having water as a solvent, an active stoichiometric
ratio of Li.sub.2S.sub.4, and a sulfur concentration of 10
moles/liter (molar) sulfur. The precursor chemicals Li.sub.2S and
elemental sulfur are used in proper proportion to yield an active
stoichiometric ratio of Li.sub.2S.sub.4. In addition to the
precursor chemicals, Catholyte #1 also contains, dissolved therein,
an additional basic lithium salt, specifically 0.5 molar LiOH.
The catholyte was prepared in a 25 mL volumetric flask inside a
main glove box filled with argon gas (i.e., an inert gas), the
glove box having oxygen concentration of less than 5 ppm (i.e., the
environment in which the catholyte is made is substantially devoid
of molecular oxygen). The required amount of lithium hydroxide
(reagent grade, Sigma Aldrich) was weighed in a different glove box
filled with dry argon having less than 2 ppm of moisture and then
was transferred to the main glove box used for catholyte
preparation. Deionized water was boiled and transferred to the same
glove box in a closed container. Inside the glove box, argon gas
was bubbled through the container in order to remove remaining
traces of oxygen. The required amount of Li.sub.2S (Sigma Aldrich,
99% purity) was determined from the reaction 8 Li.sub.2S+3
S.sub.8.fwdarw.8 Li.sub.2S.sub.4 and mixed with the lithium
hydroxide, and placed in the flask. Next, 10 mL of the deionized
and deoxygenated water was added to the flask, and the mixture was
stirred for 30 minutes. Based on the aforementioned stoichiometric
reaction and Li.sub.2S.sub.4 as the desired active stoichiometric
ratio, the required amount of sulfur (Sigma Aldrich, reagent grade,
purified by sublimation) was added to the mixture. Then, deionized
water was added to the mixture up to the 25 mL mark, the flask was
tightly sealed (to avoid active sulfur loss in the form of H.sub.2S
gas), and the mixture was stirred overnight. Next day, the stir bar
was removed, water was added up to the 25 mL mark, and the solution
was stirred for another hour. The obtained solution was reddish
orange and did not contain any visible solids. Conductivity of the
prepared catholyte was measured using a conductometric cell
(Radiometer Analytical S.A., France) with two platinized platinum
electrodes. The obtained specific conductivity value is high (i.e.,
greater than 10.sup.-2 S/cm) and specifically the measured value
was 0.1 S/cm.
Example 2
This example pertains to the preparation and conductivity
measurement of a second active aqueous sulfur catholyte (i.e.,
Catholyte #2) having water as a solvent, an active stoichiometric
ratio of Li.sub.2S.sub.4, and a sulfur concentration of 12
moles/liter (molar) sulfur. Similar to the procedure described in
Example #1, the precursor chemicals Li.sub.2S and elemental sulfur
were used to effect the active Li.sub.2S.sub.4 active
stoichiometric ratio. The catholyte was devoid of salts (e.g.,
additional lithium salts) other than those used to generate the
active stoichiometric ratio of Li.sub.2S.sub.4. In particular, the
catholyte was devoid of supporting lithium salts or basic lithium
salts.
The catholyte was prepared in a manner similar to that for
Catholyte #1, as described above in Example 1. Required amounts of
the precursor chemicals (sulfur and Li.sub.2S) were mixed together,
placed in a 25 mL volumetric flask, and covered with deionized and
deoxygenated water up to the 25 mL mark. Notably the water is
deoxygenated prior to contacting the precursor chemicals. The flask
was tightly sealed and the contents were stirred. The mixture
quickly turned reddish orange and its temperature rose
significantly. Dissolution (via, in part, hydrolysis) occurred
quickly, and much faster than during preparation of Catholyte #1
since the presence of LiOH slows down the rate of Li.sub.2S
hydrolysis. After stirring the mixture overnight, a clear reddish
orange liquid was obtained. Thereafter the stir bar was removed,
water was added up to the 25 mL mark, and the solution was stirred
for another hour. The conductivity of the prepared catholyte was
measured in a manner similar to that described in Example 1, and a
specific conductivity value of 8.times.10.sup.-2 S/cm was
obtained.
Example 3
This example pertains to the preparation of a third active aqueous
sulfur catholyte (i.e., Catholyte #3) having water as a solvent, an
active stoichiometric ratio of Li.sub.2S.sub.4, and a sulfur
concentration of 17 moles/liter (molar) sulfur. Similar to that
described in Example #1, the precursor chemicals Li.sub.2S and
elemental sulfur are used to effect the active Li.sub.2S.sub.4
active stoichiometric ratio. The catholyte is devoid of salts
(e.g., additional lithium salts) other than those used to generate
the active stoichiometric ratio of Li.sub.2S.sub.4. In particular,
the catholyte is devoid of supporting lithium salts or basic
lithium salts.
In order to prepare a catholyte with the highest possible active
sulfur content (in the form of Li.sub.2S.sub.4), enough sulfur and
Li.sub.2S were mixed to prepare 20M sulfur having an active
stoichiometric ratio of Li.sub.2S.sub.4. Then the same procedure as
used in Example 2 was followed. After stirring overnight, the
solution was not clear and contained undissolved solids. The
solution was filtered through a glass microfiber GF/A filter and
the clear filtrate (i.e., clear catholyte solution) was analyzed
for total dissolved sulfur content using a method that was
described in the article by G. Schwarzenbach, A. Fischer in Heir.
Chim. Acta 43, 1365-1390 (1960), and entitled Die Aciditat der
sulfane and die zusammensetzung wasseriger polysulfdlosungen. The
article, and specifically the method for determining sulfur
concentration, is hereby incorporated by reference. In particular,
the dissolved sulfur-containing species were oxidized to sulfate,
which was then titrated by barium perchlorate in the presence of
Thorin indicator. The determined sulfur concentration in the
catholyte (i.e., sulfur molarity) was 17.25M sulfur (i.e., greater
than 17 molar sulfur).
Example 4
This example pertains to the preparation and conductivity
measurement of a fourth active aqueous sulfur catholyte (i.e.,
Catholyte #4) having water as a solvent, an active stoichiometric
ratio of Li.sub.2S, and a sulfur concentration of 3 moles/liter
sulfur (3 molar sulfur). In this example, the precursor chemical
was solely Li.sub.2S, and the catholyte was devoid of additional
salts (e.g., additional lithium salts). In particular, the
catholyte was devoid of supporting lithium salts or basic lithium
salts.
The required amount of Li.sub.2S was placed in a volumetric flask
and deionized and deoxygenated water (as described above) was added
to the 25 mL mark. The mixture quickly turned reddish orange and
its temperature rose significantly. The mixture was stirred
overnight, then the stir bar was removed, water was added up to the
25 mL mark, and the solution was stirred for another hour. The
resulting liquid was clear and had a reddish orange color. This
experiment indicates that the solubility of Li.sub.2S in water is
quite high.
The conductivity of the prepared catholyte containing products of
Li.sub.2S hydrolysis was measured in a manner similar to that used
in Example 1, and an exceptionally high value of 2.times.10.sup.-1
S/cm was obtained (i.e., greater than 10.sup.-1 S/cm).
By this expedient, and as described herein below in Example #11,
water dissolved Li.sub.2S may be used as a source of active Li for
insertion (e.g., intercalation), for instance, for the purpose of
charging alternative anodes such as carbon-based intercalation
materials and other materials that are devoid of active lithium
upon cell fabrication, including those instances in which the cell
is assembled in a discharged state (e.g., a fully discharged
state). Moreover, the high solubility and fast dissolution kinetics
of Li.sub.2S in water eliminates or significantly reduces problems
associated with precipitation of Li.sub.2S discharge product on the
cathode surface (or inside the cathode pore space) where it can
adversely effect cell performance, especially cycle life.
Example 5
This example pertains to the preparation and conductivity
measurement of a protic non-aqueous active sulfur catholyte (i.e.,
Catholyte #5) having alcohol as a solvent (specifically methanol),
an active stoichiometric ratio of Li.sub.2S.sub.4, and a sulfur
concentration of 6 moles/liter (molar) sulfur. Similar to the
procedure described in Example #1, the precursor chemicals
Li.sub.2S and elemental sulfur were used to effect the active
Li.sub.2S.sub.4 active stoichiometric ratio. The catholyte was
devoid of salts (e.g., additional lithium salts) other than those
used to generate the active stoichiometric ratio of
Li.sub.2S.sub.4. In particular, the catholyte was devoid of
supporting lithium salts or basic lithium salts.
The required amount of sulfur and Li.sub.2S precursor chemicals
were placed in a 25 mL volumetric flask, and the rest of the
operations were similar to those described in Example #2, except
that methanol was used instead of water. The resulting protic
non-aqueous catholyte was clear and had a reddish orange color. Its
conductivity was measured to be 1.1.times.10.sup.-2 S/cm.
Electrochemical Testing of Li/S Cells
The following examples provide details illustrating electrochemical
testing of Li/S cells in accordance with the present invention.
These examples are provided to exemplify and more clearly
illustrate aspects of the present invention and are in no way
intended to be limiting.
Preparation of Cathode Materials:
Carbon based electron transfer mediums were used as the cathode
(i.e., carbon based cathodes). Specifically, a porous carbon paper
matrix (Lydall Technical Papers, Rochester, N.Y.) coated with a
carbon binder slurry of 70 (wt) % acetylene black and 30% PVdF,
with a dry slurry weight of about 1.3 mg/cm.sup.2 was used.
Lead based electron transfer mediums used as the cathode (i.e.,
lead based cathodes) were prepared by electroplating lead as a
surface coating onto a core substrate of nickel (Ni ExMet type 5Ni
5-050 from DEXMET Corp.). The lead was coated from a solution
having the following composition: 200 g/L Lead (II) Carbonate,
PbCO.sub.3, 100 mL/L Tetrafluoroboric acid, HBF.sub.4, 15 g/L Boric
acid, H.sub.3BO.sub.3, 5 g/L Hydroquinone.
A rectangular piece of lead foil with a thickness of 1.6 mm was
used as an anode during electroplating. The current density was 5
mA/cm.sup.2 and the thickness of the deposited lead coating was
approximately 30 .mu.m.
Cobalt based electron transfer mediums used as the cathode (i.e.,
cobalt based cathodes) were prepared by electroplating cobalt onto
a copper substrate (Cu ExMet 1.5 Cu 5.5-OSOF1 from Delker Corp.)
from a solution having the following composition: 450 g/L Cobalt
Sulfate Heptahydrate, CoSO.sub.4.7H.sub.2O, 15 g/L Sodium Chloride,
NaCl, 40 g/L Boric acid, H.sub.3BO.sub.3.
A graphite plate with a thickness of 6 mm served as an anode during
electroplating. Electroplating was performed at a temperature of
35-40.degree. C. at a current density of 20 mA/cm.sup.2 and the
resulting thickness of the deposited cobalt was approximately 25
.mu.m.
Example 6
Determination of Potential Window for Li/S Cell Operation Using
Cyclic Voltammetry
Cyclic voltammetry experiments were performed in hermetically
sealed glass cells with plastic covers. The cells were assembled
and filled with polysulfide-containing aqueous electrolyte in a
glove box containing argon gas with an oxygen concentration of less
than 5 ppm (i.e., substantially devoid of molecular oxygen).
Aqueous electrolyte containing polysulfides had a composition of 4M
Sulfur and an active stoichiometric ratio of Li.sub.2S.sub.4. For
comparison testing, an aqueous electrolyte based on lithium
sulfate, which did not contain active sulfur species, was also
prepared. The pH of the second electrolyte was adjusted to the pH
of the first electrolyte (pH 12) by addition of LiOH.
The working electrode was either a 1 cm.times.1 cm square carbon
paper-based cathode or a 1 cm.times.1 cm square lead cathode as
described above. The working electrode was located between two
protected lithium electrodes (as described herein above) serving as
counter-electrodes in the cell. Lithium foil area was 22
mm.times.22 mm in each of the counter electrodes. Working electrode
potential was measured vs. an Ag/AgCl reference electrode and then
was recalculated into potentials vs. a Li/Li.sup.+ electrode. The
cyclic voltammetry curves were measured using a VMP-3
potentiostat/galvanostat (Bio-Logic Science Instruments, France) at
a scan rate of 0.5 mV/s.
FIG. 8 shows cyclic voltammetry curves for a carbon electrode in
aqueous electrolytes with and without dissolved polysulfides. The
cyclic voltammetry curves have several characteristic regions.
(Region A is magnified on the right graph of FIG. 8). The
voltammetry curve of the sulfate electrolyte allows determination
of the hydrogen evolution potential (cathodic current in region A
at potentials below 2.0V) and oxygen evolution (anodic current in
region D at potentials above 3.8V) on the surface of the carbon
electrode (i.e., carbon based electron transfer medium). Comparison
of voltammetry curves for the two electrolytes indicates that
cathodic currents in region A for the polysulfide electrolyte are
attributed to electroreduction of sulfur-containing species. The
right graph clearly shows that in order to minimize the
contribution of the side reaction (hydrogen evolution) in the cell
with a carbon based electron transfer medium serving as cathode,
the cell discharge voltage should not be allowed to go below
approximately 2.0V in certain embodiments. Region B on the
polysulfide electrolyte curve corresponds to the electrooxidation
of sulfur-containing species. Highly oxidized sulfur-containing
species can decompose forming elemental sulfur, which can also be
formed directly at high enough positive potentials. Deposition of
insulative sulfur on the carbon surface leads to a decrease in
current (region C) and large hysteresis on the cyclic voltammetry
curve at potentials over 2.7-2.8V.
FIG. 9 demonstrates that the lead based electrode has a
significantly greater overpotential for hydrogen evolution than the
carbon electrode. Therefore, the use of lead on the electron
transfer medium allows for an increase in the potential window for
Li/S cell operation.
FIG. 10 shows cyclic voltammetry curves in a wide potential range
for carbon and lead positive electrodes (i.e., cathodes) in
electrolytes containing dissolved polysulfides. These curves
demonstrate that the prepared lead electrode had a better rate
capability than the carbon electrode.
Example 7
Cyclic Performance of Li/S Cells With Carbon Cathode
Cyclability tests were performed in hermetically sealed Li/S cells
having two compartments: a protected lithium anode compartment and
an aqueous sulfur cathode compartment. A substantially impervious
glass-ceramic membrane, as described herein above, was fitted into
the cell by means of two Kalrez o-rings such that the membrane was
exposed to the aqueous catholyte from the cathode side and to the
non-aqueous electrolyte from the anode side. The anode compartment
was assembled in an argon-filled dry box and contained a 125
.mu.m-thick lithium foil from FMC Lithium Corp in a shape of a disc
with a diameter of 1/2'' pressed onto a nickel foil current
collector, a 1''.times.1'' square 150 .mu.m-thick glass-ceramic
solid electrolyte membrane from Ohara Corp. (Japan), and Celgard
2400 microporous separator in a shape of a disc with a diameter of
9/16''. The separator was impregnated with a non-aqueous
electrolyte containing 1 M of LiTFSI salt in 1,3-dioxolane and
placed between the Li foil surface and the glass-ceramic
membrane.
After the anode compartment was built, it was transferred to the
dry box filled with oxygen-free argon, where the cathode
compartment was assembled, filled with aqueous catholyte and
hermetically sealed. The aqueous catholyte (Catholyte #2) contained
12M S as Li.sub.2S.sub.4 in water. A 9/16''-diameter disc of
microporous Celgard 3401 separator was impregnated with the
catholyte and placed on the surface of the glass-ceramic protective
membrane. The carbon cathode described above was cut in a shape of
a 1/2''-diameter disc and placed on top of the Celgard 3401
separator layer. A 1/2''-diameter stainless steel disc was used as
a cathode current collector. The components of the cathode
compartment were kept in contact with a stainless steel spring. The
assembled cell exhibited an open circuit voltage of greater than
2.5 volts.
Cell cycling was performed using a Maccor battery tester. The
cycling procedure was as follows. The first discharge at a current
density of 1 mA/cm.sup.2 to the cut-off voltage of 2.1V was
followed by a charge at 0.5 mA/cm.sup.2 to the capacity equal to
the previous discharge capacity. The second discharge was equal to
the previous charge capacity. Then the cell was cycled at a
constant capacity corresponding to the second discharge. The charge
cut-off voltage was set to 2.8V.
FIG. 11 shows the cycling performance of the Li/S cell. The cell
exhibited good cyclability and over 100 cycles were achieved. This
is the first known example of a rechargeable aqueous Li/S cell
having dissolved active sulfur species.
FIG. 12 shows charge and discharge voltage profiles. A high
round-trip efficiency value of 87% was calculated from average
discharge and charge voltages.
Example 8
The Li/S cell and catholyte composition were the same as described
in Example #7. However, in this case the carbon based cathode and
the stainless steel cathode current collector were immersed
overnight in a solution with the same composition as the Li/S cell
catholyte, 12M S having an active stoichiometric ratio of
Li.sub.2S.sub.4 in water. The goal of this pre-treatment was to
avoid consumption of active sulfur species by the reaction with the
cathode and the current collector in the assembled cell. After
storage in the catholyte solution overnight, the cathode and the
current collector were removed and rinsed in sequence with 0.5M
LiOH, water, toluene, and methanol, and then dried. It was found
that the pre-treatment in a sulfur-containing solution greatly
improved the wettability of the carbon electrode with catholyte
during cathode compartment filling. The cycling procedure included
a discharge at a current density of 1 mA/cm.sup.2 to the cut-off
voltage of 2.0V and a charge at 0.5 mA/cm.sup.2 to the capacity
equal to the previous discharge capacity. The charge cut-off
voltage was set to 2.8V.
Voltage-time discharge/charge profiles and delivered capacity vs.
cycle number plots are shown in FIG. 13. Under described test
conditions, the cell demonstrated good cycle life of over 50 cycles
with small capacity fade.
Example 9
The cell and catholyte composition and cycling procedure were the
same as described in Example #7. However, instead of a carbon
electrode with a nickel current collector, a lead electrode with a
lead current collector was used. The electrode and the current
collector were pre-treated in the catholyte solution as described
in Example #8.
As seen in FIG. 14, which shows voltage-time discharge/charge
profiles and delivered capacity vs. cycle number plots, Li/S cells
using the lead based cathode can be cycled at a high areal capacity
of approximately 12 mAh/cm.sup.2.
Example 10
The cell and catholyte composition were the same as described in
Example #7. The cycling procedure was the same as described in
Example #8. However, instead of a carbon electrode with a nickel
current collector, a cobalt electrode described above with a
cobalt-electroplated copper current collector was used. The
electrode and the current collector were pre-treated in the
catholyte solution as described in Example #8.
Voltage-time discharge/charge profiles and delivered capacity vs.
cycle number plots are shown in FIG. 15. Under described test
conditions, the cell demonstrated several discharge-charge
cycles.
Example 11
The cell was similar to the one described in Example #7. However,
in this case a carbon anode was used instead of a lithium metal
anode, and the aqueous catholyte contained 3M Li.sub.2S (Catholyte
#4). The anode was a commercial carbon electrode comprising
synthetic graphite on a copper substrate and was similar to carbon
electrodes commonly used in lithium-ion batteries. The non-aqueous
electrolyte interlayer contained 1M of LiTFSI salt dissolved in the
mixture of ethylene carbonate and dimethyl carbonate (1:1 by
volume). The assembled cell with the following structure: carbon
anode/non-aqueous electrolyte/glass-ceramic membrane/aqueous
Li.sub.2S catholyte/carbon cathode exhibited an open circuit
voltage of -0.63V.
First, the cell was galvanostatically charged at a current density
of 0.1 mA/cm.sup.2 for 20 hours. At the end of the charge, the cell
voltage reached approximately 2.4V. Then, the cell was discharged
at 0.1 mA/cm.sup.2 to a voltage cut-off of 2.1V. The same
charge/discharge procedure was used for further cycling: the cell
was charged at 0.1 mA/cm.sup.2 for 20 hours and then discharged at
0.1 mA/cm.sup.2 to 2.1V.
FIG. 16 demonstrates that a cell employing a carbon anode and an
aqueous electrolyte containing Li.sub.2S can work reversibly. This
is the first known example of an aqueous solution containing
lithium sulfides or polysulfides being used as a source of Li
cations for charging of a carbon anode. Therefore, aqueous
catholytes containing active sulfur species can be used in
combination with lithium intercalation compounds in rechargeable
lithium-sulfur batteries.
Example 12
The cell, pre-treated cathode and cycling procedure were the same
as described in Example #10. However, the catholyte contained 6M S
having an active stoichiometric ratio of Li.sub.2S.sub.4 in
methanol (Catholyte #5, described above).
Voltage-time discharge/charge profiles and delivered capacity vs.
cycle number plots for Li/S cells with a cobalt cathode and
methanol-based sulfur-containing catholyte are shown in FIG. 17.
Under the described test conditions, the cell demonstrated several
discharge-charge cycles. This is the first known example of a
rechargeable Li/S cell with a catholyte based on a protic
nonaqueous solvent.
Conclusion
Various embodiments of the invention have been described. However a
person of ordinary skill in the art will recognize that various
modifications may be made to the described embodiments without
departing from the scope of the claims. Accordingly, the present
embodiments are to be considered as illustrative and not
restrictive, and the invention is not to be limited to the details
given herein.
* * * * *
References